A mollusc in secret (4)

I believe the answer is:

clam

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'a mollusc' is the definition.
(clam is a kind of mollusc)

I don't understand how the rest of the clue works.

(Other definitions for clam that I've seen before include "Aquatic creature" , "Large marine bivalve" , "ringing noise produced by > very reticent type" , "Mollusc, one proverbially reticent" , "shy person" .)

I've seen this clue in The Times.

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