Bivalve mollusk (4)
'bivalve mollusk' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for clam that I've seen before include "Mollusc, one proverbially reticent" , "Burrowing marine mollusc living in sand or mud" , "Secretive person - mollusc" , "A silent type" , "Shellfish - secretive type" .)
(Other definitions for mussel that I've seen before include "Edible bivalve mollusc, common in sea" , "Edible bivalve mollusc, sticks to rocks" , "Edible shellfish" , "Black marine bivalves" , "Seafood item" .)