Chowder shellfish (4)
I believe the answer is:
clam
'chowder shellfish' 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 "Aquatic creature" , "Large shellfish with hinged shell" , "Burrowing marine mollusc living in sand or mud" , "shy person" , "Proverbially reticent marine mollusc" .)