ACROSS
6/7. <clumsy> ALL THUMBS = AL + LT + HUM + BS
9. <cast> THROW = HR inside TOWN
10. F1 + TIN = FIT IN <be socially accepted>
11. <excessively> OU THE WAZOO = ⭯TOUT⭯ + HE + WA + ZOO
14. (-s)U(-r)P(-a)S(-s)E(-d) + T = UPSET <underdog victor>
16. <cross swords with> FENCE <thief's accomplice>
17/18. <stone> TIGER'S EYE = T + IE outside GEYSER*

DOWN
1. GAB outside (STROP U) = GASTROPUB <eatery>
2. LEFT* outside OR = FLORET <bit of broccoli>
3. <sulk> STEW = ←WETS←
4. CUR + FEW = CURFEW <restraint when it comes to nighttime activities>
5. <be against> ABUT = ←TUBA←
8. ROOKNEEDS* = SNOOKERED <in an awkward game position>
12. HOED outside TB = HOTBED <breeding ground>
13. (-main)Z IN GER(-many) = ZINGER <return fire?>
15. <don't have> SKIP = S + KIP
16. FI(-r)ST = FIST <duke>