Tentacled mollusc (5)
'tentacled mollusc' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for octopus that I've seen before include "Cop-outs (anag.)" , "Tentacled cephalopod" , "Cops out (anag.)" , "One well-armed" , "Ocean-dweller" .)
(Other definitions for squid that I've seen before include "Cephalopod mollusc" , "Edible eight-arm mollusc" , "Sea creature" , "Large cephalopod" , "Item of seafood" .)