Theme: Got your boarding pass? - Airlines.
17A. Law of the jungle, in the Old West: FRONTIER JUSTICE. Frontier Airlines.
23A. 1867 territorial acquisition dubbed "Seward's Folly": ALASKA PURCHASE. Alaska Airlines.
35A. 1973 Helen Reddy chart-topper: "DELTA DAWN". Delta Air Lines, a legacy airline.
49A. Melted yellow square on a burger: AMERICAN CHEESE. American Airlines.
55A. R. Kelly hit whose last title word is aptly rhymed with "sky" in the lyrics ... and a hint to the starts of the other four longest puzzle answers: I BELIEVE I CAN FLY, from the soundtrack to the 1996 film Space Jam.
Argyle here. My first flight was on a DC-3, Mohawk Airlines. On another note, nice puzzle.
1. "Hometown Proud" market chain: IGA. (Independent Grocers Alliance) Our town's IGA closed in 1999.
4. First-string squad: A TEAM
9. Actor McGregor: EWAN
13. Election Day mo.: NOVenber
14. "Lord Jim" author Joseph: CONRAD
16. Field-plowing site: FARM
20. Freezer cubes: ICE
21. Top-notch: A ONE
22. Texter's gratitude: THX. (Thanks)
28. Old King Cole was a merry one: OLD SOUL
29. Reason for unbalanced books: ERROR
30. Southeast Asian language: LAO
31. __ hasty retreat: BEAT A
34. Walk-__: bit parts: ONs
38. CBS forensic series: CSI. (Crime Scene Investigation)
40. African virus: EBOLA
41. ID on an auto title: VIN. (vehicle identification number)
44. Causes pain: HURTS
47. Refreshing break: RESPITE
52. "Let me think ... ": "HMM ... "
53. Smell: ODOR. Burning rubber? That's just me thinking.
54. U.S. intelligence org.: NSA. (No Such Agency)
61. Extremely: VERY
62. On __ knee: kneeling: BENDED
63. Suffix with human: OID. (humanoid)
64. Celtic language: ERSE
65. Pulled into piles, as leaves: RAKED
66. Hi-__ graphics: RES. (High-resolution)
1. With one flat, musically: IN F
2. Largest living primate: GORILLA
3. Guacamole fruit: AVOCADO
4. Perform on stage: ACT
5. You, to Yves: TOI. French.
6. Opposite of WSW: ENE
7. Pianist Claudio: ARRAU
8. College focus: MAJOR
9. Little newt: EFT
10. Serves food to: WAITS ON
11. Bow-and-arrow wielders: ARCHERS
12. Santa Fe's st.: NMEX. (New Mexico)
15. Dummy: DUNCE
18. Monster's loch: NESS
19. 61-Across: Ger.: SEHR
23. "You've Got Mail" ISP: AOL
24. NBA great Bryant: KOBE
25. "The Clan of the Cave Bear" author Jean: AUEL. An old crossword favorite brought out of retirement.
26. Place setting disc: PLATE
27. Elvis __ Presley: ARON. It's what is on the birth certificate, alright? Anons keep away.
32. Bill with cocktails: TAB
33. Decorate: ADORN
35. Like desperate circumstances: DIRE
36. Baldwin of "30 Rock": ALEC
37. Laundry: WASH
38. __ of commerce: CHAMBER
39. Hot seasons: SUMMERS. Now if it were Nice hot seasons ....
41. Competes to get: VIES FOR
42. "That's not true!": "IT'S A LIE!"
43. Bridal bio word: NÉE
45. "Star Trek: T.N.G." counselor: TROI
46. Flip of a hit single: SIDE B
48. Teller's comedy partner: PENN
50. Tarp, e.g.: COVER
51. Hockey venue: ARENA
52. Bee home: HIVE
56. Soapmaking supply: LYE
57. "Beats me," in texts: [IDK]. [I Don't Know]
58. Mediocre grade: CEE
59. Tack on, as tax: ADD
60. Golf hole meas.: YDs. Yards