One reluctant to describe shellfish? (4)
I believe the answer is:
clam
I believe this clue is a double definition.
'one reluctant to describe' is the first definition.
(clam can mean someone reluctant to speak)
'shellfish?' is the second definition.
(clam is a kind of shellfish)
(Other definitions for clam that I've seen before include "Large shellfish with hinged shell" , "Mollusc with tightly-closed shell" , "Burrowing marine mollusc living in sand or mud" , "Reticent shellfish?" , "Bi-valve with firmly-closed shell" .)