ACROSS
1. <where performers might learn> CONSERVATOIRE = OVERREACTIONS*
9. <shockingly sexy> LURID = LID outside _UR_
10. <bluffer's expression> POKER FACE = (PERF outside OK) ACE
11. <shabby> MOTH-EATEN = MO (THEATRE - R) N
13. J OUST = JOUST <duel>
14. ANY outside UNIT* = ANNUITY <yearly payout>
16. <turbochargers> SOUPS UP = (SUP SUP) outside O_
18. <this clears view> EYEWASH = _E (YEW ASH)
20. ←E NERF← MY = FRENEMY <close rival>
21. <hostile> SPIKY = "SPY + KEY"
22. (...w)AY ATOLL A H(a...) = AYATOLLAH <religious leader>
24. <musician> GUITARIST = GUT outside I + SITAR*
25. <it's over a foot> ANKLE = L inside _ANKE_
27. <group of evildoers> ROGUES GALLERY = (REGULARLY GOES)*

DOWN
1/25. "KHAKI" = CAR KEY <it can cause ignition>
2. _N _U _D _G _E = NUDGE <prod>
3. (PAY THEM)* = EMPATHY <fellow feeling>
4. <northern raiders> VIKING = V (I KINGS)
5. "TERROR JEWEL" = TERAJOULE <energy equivalent to several hundred tonnes of TNT>
6. I SUNTAN* HELL = IN A NUTSHELL <briefly>
7. <utilise> EXERT = -P, EXPERT
8. LAMB inside CAKE = CLAMBAKE <beach party>
12. <perhaps marinading> TENDERISING = INGREDIENTS*
15. INANE outside (YMCAS - M) = IN ANY CASE <regardless>
17. <takes note of> PAYS HEED = PAY SHE E_D
19. <trial> HEARING = HARING outside E
20. (FLAT ARC)* = FRACTAL <repeating geometric shape>
21. <honey> SUGAR = ←RAGUS←
23. <roundish> OVATE = O VAT + _E