A marine crustacean (7)
I believe the answer is:
lobster
'a marine crustacean' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lobster that I've seen before include "One showing claws" , "Marine creature" , "one to loft a > shellfish?" , "Large marine decapod" , "Tommy no longer" .)