ACROSS
 1. (L AND M) + ASSES = LANDMASSES <Australia and the Americas?>
 7. "PACKED" = PACT <agreement>
 9. <daring> HEROIC = O inside HER IC
10. <piece for choir> ORATORIO = OR A TRIO outside O
11. RADICAL* outside R = RAILCARD <reduced-fare pass>
12. INS IT + U = IN SITU <right there>
13. ←BRAG← = GARB <outfit>
15. <deemed> CONSIDERED = CON SIDE RED
17. GUNS* + LASSES = SUNGLASSES <beachwear>
20. <ancient city> URUK = U_R UK
21. (ARCHEOLOGISTS - COLOSSI)* = GATHER <hoard>
23. <material> RELEVANT = RE LEVANT
25. <tall tale> PORKY PIE = Y inside (PORK PIE)
26. <IT person> TECHIE = TEC H_ IE
27. <pair> DYAD = Y_ inside DAD
28. "SAY SHELLS" = SEYCHELLES <somewhere in the Indian Ocean>

DOWN
 2. <leader of youth group> AKELA = (.n)AKE LA(m.)
 3. DOG outside DOUBLE* = DOODLEBUG <bomb>
 4. <old-fashioned> ARCHAIC = ARCH ←CIA←
 5. (A NREDY CROSSWORD)* = SWORD AND SORCERY <fantasy subgenre>
 6. <language> SWAHILI = (LAW IS)* outside HI
 7. <devoted> PIOUS = P + IOUS
 8. COIN T_ R_ EAU = COINTREAU <orange liqueur>
14. <flattering> ADULATORY = A inside A DUL_ TORY
16. <undecided> EQUIVOCAL = EQUAL outside (IV O + C_)
18. A PROP + OS = APROPOS <fitting>
19. <mark> SPLOTCH = PLOT inside SCH
22. <increased> HIKED = (.s)HI KED(g.)
24. NOSE outside _I_ = NOISE <racket>