Silent one (4)
I believe the answer is:
clam
'silent one' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for clam that I've seen before include "Large shellfish with hinged shell" , "Bi-valve with firmly-closed shell" , "Burrowing marine mollusc living in sand or mud" , "Mollusc, one proverbially reticent" , "Mollusc with firmly-closed shell" .)