ACROSS
 8. _L (BINDING RARELY)* = LENDING LIBRARY <books here>
 9. AC ES = ACES <brilliant>
10. HOOT NANNY outside E = HOOTENANNY <music event>
11. LAUNCH, L→R = RAUNCH <vulgarity>
12. <prizewinning poet> LAUREATE = L AUREATE
14. "OWED" = ODE <poem>
15. JA inside DEV + _U = DEJA VU <memory issue>
17. <Roman god> SATURN = SAT + URN
19. <came up against> MET <London police>
20. (EARL + DUTY)* = ADULTERY <infidelity>
22. ←NO LAVA← = AVALON <island>
24. (ORAL DEBATE)* = ELABORATED <went into more detail>
26. <there's money in this> TILL <work on farm>
27. <improvised> EXTEMPORANEOUS = EXTRANEOUS outside POEM*

DOWN
 1. BADE outside LOCK = BLOCKADE <siege>
 2. <agreement> UNISON = ON under UNIS
 3. <sailor's guide> LIGHTHOUSE = (ISLE THOUGH)*
 4. <mean> IGNOBLE = GOBLIN* _E
 5. H IDE = HIDE <skin>
 6. MAM inside AREN'T = ARMAMENT <military equipment>
 7. <dark-haired> BRUNET = BT outside RUNE
13. <good> UPSTANDING = UP STANDING
16. AD LIED outside BB = AD-LIBBED <made stuff up>
18. ROOTLES S_ = ROOTLESS <vagabond>
19. MY S TRY outside _E_ = MYSTERY <puzzle>
21. <opulent> DELUXE = (DEUX outside L) _E
23. <write> AUTHOR = (RHO + TAU)*