Bivalve mollusk (4)
I believe the answer is:
clam
'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" .)