ACROSS
 7. (.e)ARE NA(r.) = ARENA <concert venue>
 8. (A THE) + ART = AT HEART <essentially>
10. KING outside ILL = KILLING <murder>
11. <groups of wolves> PACKS = "PAX"
12. <arranging> ORCHESTRATING = HECTOR'S* RATING
15. B UTTER FINGERS = BUTTERFINGERS <klutz>
17. C LOUT = CLOUT <influence>
18. <percussion> TIMPANI = (IN A PIT)* outside M
20. <grassland> PASTURE = PAST URE
21. <clipped> SHORN = S_ + HORN

DOWN
 1. ←(.a)B MOCK CAB(b.)← = BACK-COMB <hair treatment>
 2. OCEANOLOGIST* = GEOLOCATIONS <latitude and longitude coordinates>
 3. B_ A_ R_ I_ = BARI <Italian city>
 4. <heard> CAUGHT = C AUGHT
 5. <minister> CHAPLAIN = CHA PLAIN
 6. <Greek characters> ETAS = ←SATE←
 9. <island groups> ARCHIPELAGOS = A RC HIP E LAGOS
13. <busy> EVENTFUL = FLUE* outside VENT
14. G AGAINST* = GAS GIANT <planet>
16. FAT HER[-e] = FATHER <pops>
17. COP + E = COPE <weather hard times>
19. "MASC" = MASK <façade>