ACROSS
 1. ROMANTICISED* = DOMINATRICES <sadistic women>
 8. <leave> PASS ON = PAS + SON
 9. LAIN after CHAP = CHAPLAIN <clergyman>
10. "FEAR CYST" = FIERCEST <most inflamed>
11. <tumbledown building> SHANTY = SHAN'T _Y
13. <thrive at the expense of> OUTCOMPETE = PET inside OUTCOME
15. <cat's paw> PAWN <hock>
17. <painter> DALI = (.o)DALI(s.)
18. <plant> SNAPDRAGON = SNAP ON outside DRAG
20. <lines of text> STANZA = STAN ZA
22. <rubber> MASSEUSE = MASSÉ + USE
24. <interrogate> QUESTION = ION after QUEST
26. A KIT AS = AKITAS <guard dogs>
27. KEYS ←GI← NATURE = KEY SIGNATURE <set of flats?>

DOWN
 1. D + _E ST RUCTION = DESTRUCTION <carnage>
 2. MONTE[-video] = MONTE <card game>
 3. →"TEC NIGH"← = NECKTIE <formalwear>
 4. <go past> TRANSCEND = T_ R_ + N inside ASCEND
 5. I (L L) = ILL <doing badly>
 6. <be> EXIST = [-s]EXIST
 7. <Roman emperor> CALIGULA = CA LI GAUL*
12. <order> ALPHABETISE = (HEAPS ALBEIT)*
14. <PR> MESSAGING = MESS AGING
16. <forester> WOODSMAN = WOOS MAN outside D
19. AMMO N_ I_ + A = AMMONIA <toxic gas>
21. C_ inside TRUE = TRUCE <armistice>
23. F inside SN + AU = SNAFU <mix-up>
25. S US = SUS <fishy>