$1 bill, in slang (4)

I believe the answer is:

clam

(Other definitions for clam that I've seen before include "Bi-valve with firmly-closed shell" , "Proverbially reticent marine mollusc" , "A silent type" , "Large marine bivalve" , "Tightly closed shellfish" .)

I've seen this clue in The New York Times.

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