Small edible crustacean (6)
'small edible crustacean' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for shrimp that I've seen before include "Seafood" , "Raising of a siege" , "Crustacean; small child" , "ingredient of cocktail" , "Small shellfish or very small person" .)
(Other definitions for prawn that I've seen before include "Small tasty crustacean, nice in cocktail" , "Cocktail ingredient" , "Bit of seafood" , "Popular oriental snack" , "Large shrimp" .)