Shellfish left in river (4)

I believe the answer is:

clam

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

I can't explain the rest of the clue.

(Other definitions for clam that I've seen before include "A silent type" , "Proverbially reticent marine mollusc" , "One is as uncommunicative as a mollusc" , "Mollusc with tightly-closed shell" , "Bi-valve with firmly-closed shell" .)

I've seen this clue in The Telegraph, The Guardian and The Sun.

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