ACROSS
 1. WATER outside I = WAITER <restaurant employee>
 5. <indicate> SIGNPOST = SIGN POST
 9. <madam, maybe> PALINDROME = (T off TRAMPOLINED)*
10. <public transport> TRAM = ←MART←
11. MISTAKEN* = MANKIEST <most unpleasant>
12. <frank> HONEST = HONE ST
13. <consumption> TUBERCULOSIS = TUBERS outside CU SOIL*
17. (COMEDIAN + LIAR)* = AMERICAN IDOL <televised competition>
20. <minister> BISHOP = ISH inside BOP
22. T_I inside REAR II = RETIARII <gladiators>
24. I ZZ after R = RIZZ <charm>
25. <miser> CHEAPSKATE = (TO KEEP A CASH)* - 0
26. ANGLES _E Y = ANGLESEY <Welsh location>
27. CURTSY, S→L = CURTLY <without ceremony>

DOWN
 2. L off LAVA + TAR = AVATAR <profile picture>
 3. <mischievous figure> TRICKSTER = (.a)TRICK'S TER(i.)
 4. RIDE outside G = RIDGE <escarpment>
 5. <gun> SHOOTER = S HOOTER
 6. <dog> GREYHOUND = →"(HAY GROUND)"←
 7. ←NO + TIP← = PITON <mountaineering equipment>
 8. S_ TARS HIP = STARSHIP <sci-fi vehicle>
14. <stores> BACKPACKS = B (P_ inside ACK-ACKS)
15. <one transporting crude> OIL TANKER = _L_T_A_ inside OINKER
16. (I'M A BIT)* ON = AMBITION <drive to succeed>
18. <daycare> NURSERY = N SURREY*
19. PI STOL[-e] = PISTOL <firearm>
21. <greenish-brown> HAZEL = HAZE + L
23. TO PIC = TOPIC <subject matter>