ACROSS
 1. →"SHACK BLEEP"← = BLACK SHEEP <misfit>
 7. <collide with> BUMP = BUM + P
 9. <dig> ADMIRE = A DIRE outside M
10. <ripe> PUNGENT = PUN + GENT
11. <best> ACME = ACE outside _M
13. <diver> KINGFISHER = (GF IS + HER) after KIN
14. <average> MEDIOCRE = MEDIC RE outside O
16. _OSSI_ + AN = OSSIAN <alleged Gaelic poet>
18. PE NUT outside A_ = PEANUT <pulse>
19. (DID ACT) _I_C_ = DIDACTIC <preachy>
20. A CROP _HO_ BIC = ACROPHOBIC <afraid of heights>
23. "MISSED" = MIST <film>
24. <river> ORINOCO = _OR_ _IN_ _OC_ _O_
25. <broadcaster> AERIAL = "ARIEL"
27. ←ELSI[-nore]← = ISLE <Sark?>
28. <game show> TASKMASTER = (TAKES SMART)*

DOWN
 2. "LEAD" = LED <came first>
 3. <restaurant in London> CRITERION = C + INTERIOR*
 4. "STAKE" = STEAK <cut>
 5. PANS inside EXE = EXPANSE <area>
 6. (FLOP + DINKY)* = PINK FLOYD <rock group>
 7. B LESS = BLESS <that's cute>
 8. MS outside A THEMATIC = MATHEMATICS <subject>
12. <brainiac> CLEVER CLOGS = VELCRO* inside C LEGS
15. <ruthless> CUT-THROAT = CU TT HR + _O_ AT
17. (ACRE + MOSSY)* = SYCAMORES <trees>
19. <questionable> DUBIOUS = ←BUD← IOUS
21. OX I_ D_ E_ = OXIDE <lime {calcium oxide}?>
22. (.i)C HAS M(e.) = CHASM <opening>
26. <wonder> AWE = A_ W_ E_