Large edible crustacean (7)
I believe the answer is:
lobster
'large edible crustacean' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for lobster that I've seen before include "potted" , "Bolster up te shellfish" , "Eight-legged shellfish, scarlet when boiled" , "Tommy no longer" , "One showing claws" .)