A sea creature shut up? (4)

I believe the answer is:

clam

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'a sea creature' is the definition.
'clam' can be an answer for 'creature' (clam is a kind of animal). I'm not sure about the 'a sea' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for clam that I've seen before include "shy person" , "Tightly closed shellfish" , "What could grip" , "Shellfish - secretive person" , "Large bivalve mollusc" .)

I've seen this clue in the Evening Standard.

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