Edible bivalve mollusc (6)
I believe the answer is:
cockle
'edible bivalve mollusc' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cockle that I've seen before include "Burrowing mollusc" , "Edible bivalve mollusc" , "occupant of shell" , "Seafood item" , "Common edible bivalve" .)