ACROSS
 1. <plantation> ARBORETUM = A R BORE + TUM
 6. <reptile> GATOR = GA TOR
 9. "HIGH JEAN" = HYGIENE <cleanliness>
10. (MY outside ONE) _E D = MONEYED <flush>
11/13. <fee on belongings left after checking out> INHERITANCE TAX {duty on estate bequeathed after dying}
14. <religious carving> TOTEM = TOTE + M
15. WHIGS outside (BAN after Z) = WHIZBANGS <fireworks>
17. <twin-hulled> CATAMARAN = CAN outside A TAMAR
19. L inside EVER = ELVER <a little fishy>
21. <stray> ERR = (.t)ERR(i.)
22. <freaked out> RHAPSODISED = (RAIDED SHOPS)*
24. <V> AGAINST = A GAINS _T
25. ←MAG← AN outside BI = GAMBIAN <from West Africa>
27. <caused> DROVE = DOVE outside R_
28. D_Y SENTRY outside E = DYSENTERY <complaint>

DOWN
 1. <not believing> ATHEISTIC = HE IS inside ATTIC
 2. <someone> BIG SHOT = BIGOT outside SH
 3. <woodland creature> ROE <eggs>
 4. T HEFT = THEFT <robbery>
 5. <large showy building> MCMANSION = ←IS← inside MCM A_ NO N
 6/26. <sweet> GINGERBREAD MAN = GIN + BREAD inside GERMAN
 7. <test limits> TRY IT ON = TRITON outside Y
 8. <brought back> REDUX = REX outside D _U_
12. <reminder> REMEMBRANCE = RE MEMBRANE outside C
15. RANTED after WAR = WARRANTED <justified>
16. <shrillness> STRIDENCY = STRIDE (C inside NY)
18. <whirlwind> TORNADO = TANDOOR*
20. VEE outside S ←IT← G = VESTIGE <evidence>
21. <antelope> ELAND = ELAN + D
23. <laments> SIGHS = "SIZE"