ACROSS
 1. <awards> OSCARS = O SCARS
 5. STAID after FIR = FIRST AID <treatment>
 9. <mythical sea horses> HIPPOCAMPI = HIP (PO CAM) + PI
10. <half-inch> NICK <cut>
11. OVERT OIL = OVERTOIL <exhaust>
13. [-c]HAMPER[-s] = HAMPER <picnic basket>
14. AC I'D = ACID <hallucinogen>
15. (I HADN'T)* USES = DIANTHUSES <pink flowers>
16. <author> IAN FLEMING = (NAMING FILE)*
18. ATE after P = PÂTÉ <liver spread>
19. AMD + RAM = AM-DRAM <acting unprofessionally>
20. + A DAM, SALE = ADAM'S ALE <water>
22. <haircut> TRIM = T_ RIM
23. I GIN inside ORALLY = ORIGINALLY <at first>
25. <drifters> PLANKTON = PLAN ON outside K_T
26. PEED inside SO = SPEEDO <swimwear>

DOWN
 2. <traevl across snow> SKI = (.u)SKI(e.)
 3. <had hopes> ASPIRED = AS PI + RED
 4. SIC KO = SICKO <sadistic person>
 5. (AS IF I AM NOT)* outside LIARI = FAMILIARISATION <acquaintance>
 6. (THE CIGARS)* = REICHSTAG <diet>
 7. <bird> TINAMOU = (.n)TINA MOU(n.)
 8. I_N_C _R_E_ + MENTAL = INCREMENTAL <bit by bit>
12. <officer> VICE ADMIRAL = (DIM CAVALIER)*
15. (MEAT + BROAD)* = DREAMBOAT <dish>
18. <emergency workers> FIREMEN = ←IF← + REME _N
21. AD + IOS = ADIOS <so long>
24. <young man> LAD = [-g]LAD