ACROSS
 1. AN ON ←MY← = ANONYM <pen name>
 5. <homeland> URHEIMAT = _UR_ _HE_ _IM_ _AT_
 9. MP inside II = IMPI <armed men in South Africa>
10. <imcomplete> CATALECTIC = TALE inside (ACT I) inside CC
11. R_H I ←OZ← CARP = RHIZOCARP <perennial herb>
12. NARCOS - RC = NAOS <sanctuary in temple>
13. "FEW GET A" = FUGHETTA <contrapuntal ditty>
15. <eel> LAUNCE = (.l)LA UNCE(r.)
17. CONCH _Y = CONCHY <one unwilling to fight>
19. AN (PO inside TILE) = ANTIPOLE <converse>
21. <"joy" rune> WYNN = ←(.u)NNY W(e.)←
23. HAD* [-m]OCKERY = AD HOCKERY <improvising>
25. <large estate> ENCOMIENDA = (INCOME, I↔E) _E_ + NDA
26. <mirror backing> _T _A IN
27. MELEE inside CAR = CAMELEER <driver through desert?>
28. <shots in billiards> CAROMS = C + ARMS outside O

DOWN
 2. <big cheese> NUMERO UNO = (ON OUR MENU)*
 3. N IRISH outside O = NOIRISH <hardboiled?>
 4. <software shortcut> MACRO = MAC + RO_
 5. <persitent athlete> ULTRAMARATHONER = (MATTERHORN + A URAL)*
 6. <flier> HELIPILOT = H + ELI + PI LOT
 7. <stranger> INCONNU = (INN outside CON) U
 8. _AV_ _I_ _S_ _O_ = AVISO <courier vessel>
14. <extinct marsupial> THYLACINE = THY LA CINE
16. <solution for e.g. pupils> COLLYRIUM = _C_O_L _L_Y _R_I_U_ M
18. <Italian ballad> CANZONE = CA + NZ ONE
20. <whirlwind> PRESTER = PR EST _E _R
22. <lady that's interfering> YENTA = YEN + TA
24. CR + ←CIA← = CRAIC <fun>