1. Lt. ___, communications officer on the original "Star Trek" → UHURA
6. Letter between rho and tau → SIGMA
11. N.H.L. tiebreakers → OTS
14. Allow to attack → LETAT
15. Epic that ends "Such was the funeral of Hector, tamer of horses" → ILIAD
16. Prefix with liberal → NEO
17. With 59-Across, "This is beyond a doubt" ... or a punny hint to 24-, 38- and 49-Across → THERESNOTWO
19. Online chats, in brief → IMS
20. Nickname for Rihanna → RIRI
21. Tummy muscles → ABS
22. Drug used to treat Parkinson's → LDOPA
24. The only person to have the opinion → AMINORITYOFONE
28. Criticize harshly → PAN
29. Unpleasant smells → ODORS
30. Temporary state → PHASE
34. Bread for a ham sandwich → RYE
35. Its products often include wordless assembly instructions → IKEA
38. Contrast → SETINOPPOSITION
42. One-named supermodel → IMAN
43. GQ or Cosmo → MAG
44. Singer/dancer/actor ___ Osmond → DONNY
45. Burton of "Star Trek: T.N.G." → LEVAR
47. Fruity drink → ADE
49. Thing bound to end in failure → NOWINSITUATION
55. Search for water, in a way → DOWSE
56. Racial justice movement, for short → BLM
57. Analyst/ex-QB Tony ___ → ROMO
58. Splendid serve → ACE
59. See 17-Across → WAYSABOUTIT
63. Apply gently → DAB
64. Succulents that soothe → ALOES
65. Exams for future docs → MCATS
66. P.D. alert → APB
67. Daredevil's deed → STUNT
68. Painter's plaster → GESSO