Theme: Wacky (absurd) cutoffs. The last three letters have been removed (reduced) from the last word in accordance with the first word indicating that something is incomplete.
17A. Schubert's eighth, aptly: UNFINISHED SYMPHony.
27A. Reference book, aptly: ABRIDGED DICTIONary.
48A. Burnout symptom, aptly: DIMINISHED INTERest.
65A. Logical principle that applies to 17-, 27- and 48-Across, aptly: REDUCTIO AD ABSURdum. Too long to summarize succinctly, you can read it if you wish.
Great theme, four grid-spanning 15 letter answers all with the same number of letters removed. Some of the fill was a bit rough, quite a few abbreviations to make it all work.
1. Needle-nosed fish: GAR.
4. Work unit: Abbr.: FT LB. Foot Pound. Lb is an abbreviation of Latin libra, the word for pound, or balance as with a scale.
8. __ Island, Florida city near Naples: MARCO. Map.
13. Put away: ATE.
14. Dickens's Heep: URIAH. Also a 70's rock band.
16. Some fitness ctrs.: YMCAS. Young Men's Christian Association.
20. Butcher's offering: LOIN. Chart of meat cuts.
21. Besides: AT THAT. My concept of this idiom is more one of concession or appeasement to smooth over an argument... But it might be right at that.
22. Garden veggie: PEA.
23. Try to be elected: RUN.
25. Sue Grafton's "__ for Lawless": L IS. A gimmie answer, even if you never have read her books. The first letter always gives it away.
36. It goes with boo, woo or yoo: HOO. Echoic emphasis.
37. Puppet dragon of early TV: OLLIE. Along with Kukla and Fran.
38. Dix follower: ONZE. French for ten and eleven.
39. Off-white shade: ECRU.
41. Banana oil, e.g.: ESTER. An artificial flavoring, Isoamyl acetate is released by a honey bee's sting apparatus where it serves as a pheromone beacon to attract other bees and provoke them to sting.
43. Pickup attachment: PLOW.
44. John Isner, a record 113 times, in a 2010 Wimbledon match: ACER. Un-returned tennis serves.
45. Lot units: ACRES.
47. Chemical suffix: ENE. Would it be so much to ask to add the phrase "indicating a carbon double-bond" to this clue? At least that way it isn't just "three random letters that you need to get from the perps".
52. California's __ Gabriel Mountains: SAN. Near LA.
53. PC brain: CPU. Central Processing Unit.
54. Govt. mortgage agency: FHA. Federal Housing Administration.
57. Brute: SAVAGE. Old French sauvage, salvage "wild, savage, untamed," from Late Latin salvaticus, alteration of silvaticus "wild," lit. "of the woods," from silva "forest, grove." Related word: sylvan, diety of the woods.
61. Engels collaborator: MARX. The Communist Manifesto. It argues that class struggles, or the exploitation of one class by another, are the motivating force behind all historical developments. Class relationships are defined by an era's means of production. However, eventually these relationships cease to be compatible with the developing forces of production. At this point, a revolution occurs and a new class emerges as the ruling one.
68. Mezzo role in Barber's "Vanessa": ERIKA. An Opera by Samuel Barber.
69. Big cats: PUMAS. Can be either black or tawny, spotted when young.
70. "So that's it!": AHA.
71. Cartridge contents: TONER.
72. Protective cover: TARP.
73. Meddle: PRY.
1. Caesarean conquest: GAUL. The Roman Empire vs ancient France.
2. Elemental ID: AT NO. Atomic number.
3. Homeowner's way to raise money, briefly: RE-FI. Refinance with cash back.
4. Pleasure: FUN.
5. Bermuda shape?: TRIANGLE. Smack in the middle of hurricane alley, the Gulf stream current cuts through it and is home to the Puerto Rico trench, the deepest point of the Atlantic. Good luck finding anything that happens to wreck there. I kind of also like the methane hydrates theory, too.
6. Supermarket need, maybe: LIST.
7. Thai money: BAHT. What, no "bread" clue on a Thursday?
8. Arcane: MYSTIC.
9. "Rehab" singer Winehouse: AMY. Now, that's ironic.
10. Dudley Do-Right's gp.: RCMP. Royal Canadian Mounted Police.
11. New England attraction, with "the": CAPE. Cape Cod, Nantucket, and Martha's Vineyard.
12. Org. concerned with asbestos stds.: OSHA. Occupational Safety and Health Administration
15. Sneaky laugh: HEH.
18. Cross letters: INRI. Iesus Nazarenus, Rex Iudaeorum. Jesus, the Nazarene, King of the Jews.
19. Picasso contemporary: DALI. We've seen enough "Persistence", here are Picasso and Self sculptures.
24. Japanese veggie: UDO. Doesn't appear particularly appetizing.
26. Station: STOP. Railroad departures.
27. In first: AHEAD.
28. Game for bowlers: BOCCI. Anglicized form of BOCCE. Tricked me.
29. Composer Ned: ROREM. I have heard of "Our Town", but otherwise not on my radar.
30. "__ Dream": "Lohengrin" aria: ELSA'S. Wagner Opera.
31. Lose on purpose: DITCH. Literally "to throw into a ditch".
32. 43-Across maker: DEERE.
33. Fjord: INLET.
34. Depleted layer: OZONE.
35. Like a 2011 model, say: NEWER.
40. "QB VII" author: URIS.
42. Aromatic evergreen: RED CEDAR.
46. Try a little of: SIP.
49. Its Hall of Fame is in Charlotte, N.C.: NASCAR. National Association for Stock Car Auto Racing
50. "For those listening __ home ...": IN AT.
51. Insensitive: NUMB. Old English "nimen" for "taken". The current meaning comes from the notion of being "taken" with cold, or shock. Oddly related to the word "nimble", quick to grasp (take).
54. Neck piece: FRET. On a guitar neck.
55. Super Bowl party order: HERO. Sub, hoagie, togo, etc.
56. Game point: AD IN. Tennis.
58. Big cheese: VIP. Very Important Person. I know you all know that, I'm just pointing out abbreviations.
59. Juillet's follower: AOUT. More French: July, August.
60. Vasco da __: GAMA.
62. Rush job notation: ASAP. As soon as possible.
63. German industrial region: RUHR.
64. Like a certain superpower: X-RAY. Superman's vision.
66. Luau strings: UKE.
67. Nile biter: ASP.