ACROSS
 1. H (PROS WERE)* outside 0 = HORSEPOWER <hp>
 7. <advertise> PLUG = L inside PUG
 9. "REAL" = REEL <section of film>
10. <cosmetic> NAIL POLISH = A inside NIL POLISH
11. <uniform> LIVERY = LIVE R_Y
12. (OCH + L) inside PINE = PINOCHLE <card game>
13. <snowmobile> SKIDOO = KID inside SO _O_
15. <supplying more weapons> REARMING = REAR + MING
17. <formal meal> LUNCHEON = (UNCLE JOHN - J)*
19. <maybe AC/DC {bi}> NONGAY = NO NAY outside G
21. <pad> FOOTSTEP = ←PETS TOO F←
23. (TO A STY) = TOASTY <cosy>
25. <combining> COALESCENT = COAL E + SCENT
26. ←W ERG← = GREW <increased>
27. L_ YON = LYON <French city>
28. UNDRESS* after ABS = ABSURDNESS <implausibility>

DOWN
 2. [-n]OBEL I SK = OBELISK <monument>
 3. SOLE outside V = SOLVE <work on crossword?>
 4. PAN THOSE outside _Y = PANTYHOSE <skintight garment>
 5. W HIP PER (_N inside SAPPERS) = WHIPPERSNAPPERS <brats>
 6. <mature> RIPEN = RIP + _E _N
 7. <bobby> POLICEMAN = ←LOP← + CINEMA*
 8. <Y> UPSILON = U_ P_ SILO _N
14. (CLOTHED AN)* = DECATHLON <event>
16. <marginal writer> ANNOTATOR = _ANNO_ _TAT_ _OR_
18. <pseudoscience> UFOLOGY = (FOOL GUY)*
20. <frank> ARTLESS = HEARTLESS - HE
22. <car manufacturer> TESLA = (.a)TES LA(r.)
24. R_ inside AGO N = ARGON <gas>