ACROSS
 1. + MUST, (GARDA'S)* = MUSTARD GAS <chemical weapon>
 7. <reference> CITE = "SITE"
 9. <conceptual framework> SCHEMA = (SCAM HE)*
10. "KNOCK TURN" = NOCTURNE <piano piece>
11. REACHER after P = PREACHER <evangelist>
13. L_ UTHER = LUTHER <religious reformer>
14. <fairy tale> CINDERELLA = RECALLED* outside IN
16. ↺OVER↺ = ROVE <range>
18. SUM outside L = SLUM <run-down area of city>
19. <skill> COMPETENCE = COMPETE ONCE - _O_
20. <everything> COSMOS = C (IS off OSMOSIS)
22. <aunt, perhaps> RELATIVE = RE L ACTIVE - _C_
24. CAD (L→, LILAC) = CADILLAC <car>
25. CHA ←ARK← = CHAKRA <yogic spiritual centre>
27. <meal> FARE = "FAIR"
28. <dimwit> DUNDERHEAD = UNDER HE inside DAD

DOWN
 2. <hostile> UNCORDIAL = (RAIN CLOUD)*
 3. T IE = TIE <formal garment>
 4. <fish> ROACH = CHAR* outside O_
 5. GEN ERA LAMER I CAN = GENERAL AMERICAN <Midwest-rooted accent>
 6. SICKLE + AVE = SICK LEAVE <time off work>
 7. <fake> COUNTERFEIT = COUNT + FREE* IT
 8. <shade> TINGE = TIN ←EG←
12. <reminder> AIDE-MÉMOIRE = ←MEDIA← EMO IRE
15. (IDOLS = SOLID*) <=< ROCK SOLID <sturdy and reliable>
17. EVER inside (VIC + SA) = VICE VERSA <the other way round>
21. O ←A HAM← = OMAHA <somewhere in Nebraska>
23. <17th-century English philosopher> LOCKE = [-c]LOCKE[-d]
26. <result of burning> ASH <wood>