ACROSS
 6. <flip> OVERSET = VERSE in OT
 7. <cleric> IMAM = ←AMI← M
 9. <charm> ENDEAR = END + EAR
10. <special language> POLARI = POLAR I
11. <African shrub> KHAT = K_ + HAT
13. <hero> SUPERMAN = P inside SURNAME*
15. (INFO outside A M) US = INFAMOUS <with bad reputation>
17. FEE D = FEED <to provide meal>
18. <test subject> LAB RAT = LA BRAT
20. <acting irresponsibly> REMISS = RE MISS
22. <venomous snakes> ASPS = AS PS
23. (SEES MAN)* = EN MASSE <all at once>

DOWN
 1. <garment> GOWN = G OWN
 2. <envy> RESENT = RE-SENT
 3. <drink> ESPRESSO = ←(.t)OSSER PSE(u.)←
 4. <on> ATOP = AT _O P
 5. <handle> TILLER = T_ ILLER
 8. "HEIR FAIR" = AIRFARE <travel money>
12. ←NAH← GARS = HANGARS <these buildings house birds>
14. <sliced beef> PASTRAMI = PAST RAM _I_
16. <not safe> AT RISK = A SKIRT*
17. <get ravenous> FAMISH = F + AMISH
19. <young person> TEEN = TEE _N
21. <cooking fat> SUET = SET outside _U