Large crustacean (7)
'large crustacean' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for lobster that I've seen before include "Marine creature" , "Eight-legged shellfish, scarlet when boiled" , "One showing claws" , "food?" , "Pincered crustacean, a delicacy" .)
(Other definitions for crab that I've seen before include "Sourpuss < to wreck" , "Seashore crustacean" , "one is a fiddler" , "Clawed shellfish" , "move obliquely" .)