ACROSS
 1. <examples> INSTANCES = IN STANCES
 6. COW _L_ = COWL <headwear>
 8. PE + A = PEA <protein-rich food>
 9. <chaos> PANDEMONIUM = (MD + PNEUMONIA)*
11. <expected> ROUTINE = "RUE TEEN"
12. <thieves> ROBBERS = _R_O_ B_B_E_-R-_S
13. <unconventional> OUTRE = OUT _R _E
15. <freethinker> LIBERTINE = LINE outside B RITE*
17. A LEANDER outside X = ALEXANDER <Greek leader>
19. HERB* outside A = REHAB <treatment for addiction>
20. <nightmarish figure> INCUBUS = IN CUBS outside U
22. <cheer> APPLAUD = A + PP LAUD
24. GRAVEST ONES = GRAVESTONES <memorials>
25. <I> EGO = EG O
26. E_Y O_T = EYOT <river location>
27. ROTTER + DAM = ROTTERDAM <city in Holland>

DOWN
 1. (.h)IMPERS ON AT ING(e.) = IMPERSONATING <at a masquerade?>
 2. ←US outside FAN← = SNAFU <omnishambles>
 3. <inform> APPRISE = ←PA← PRISE
 4. <hidden> CONCEALED = CON "SEALED"
 5. <guide> STEER = SEER outside T
 6. CROW BAR = CROWBAR <source of leverage>
 7. <perhaps sole> WHITEFISH = →"FIGHT WISH"←
10. (BOOMERS DREAMT)* = MASTER BEDROOM <sleeping area>
14. (THE OC) RACY = THEOCRACY <religious government>
16. BARNET outside MAG = BAR MAGNET <attractive bit of metal>
18. (TAMBOURINE - OUR)* = AMBIENT <sort of music>
19. ROPIEST* = RIPOSTE <counterattack>
21. <sylvan demigod> SATYR = ←AS← TYR
23. <change> AMEND = AMEN + _D