ACROSS
 1. <selfless behaviour> ALTRUISM = A L TRUISM
 6. <guns> BICEPS = ICE P_S after B
 9. O_ V_ ID = OVID <Roman poet>
10. IM ON CELLO after L = LIMONCELLO <liqueur>
11. <well-known> FAMOUS = F AMOS outside U
12. "NICK A TEEN" = NICOTINE <drug>
14. HIP POP ←TO← AMUSE S = HIPPOPOTAMUSES <animals>
17. (..l)IND US TRIAL IS ED(u..) = INDUSTRIALISED <reliant on plants?>
18. OO inside CANDLE = CANOODLE <snog>
20. NA D IS outside R = NADIRS <depths>
22. ROME inside METRIC* = MICROMETRE <unit of length>
24. <cheat> ROOK <chesspiece>
25. BE A TIT = BEAT IT <git>
26. <arrogant> ASSUMING = ASS (IM + GUN)*

DOWN
 2. <huge> LEVIATHAN = ←NAH TA IVE L←
 3. RIO outside AD = RADIO <broadcaster>
 4. <not compatible> ILL-ASSORTED = IL LASS OR TED
 5. <resistance to pivoting> MOMENT OF INERTIA = (..s)MO MEN TO FINER TIA(r..)
 6. <witticism> BON MOT = NM inside BOOT
 7. CHE ←STUNTS← = CHESTNUTS <old tales>
 8. P ONLY* = PYLON <part of the grid>
13. C HALL GREENS* = CHALLENGERS <opposition candidates?>
15. PLUTO C + RAT = PLUTOCRAT <tycoon>
16. (I WONDERED)* = EIDERDOWN <{part of 'Duck} and {Cover}'>
19. _A_S _I_D_E = A-SIDE <record's main track>
21. <cereal> DURUM = U inside DRUM