Frequent filler for a po' boy (7)

I believe the answer is:

oysters

(Other definitions for oysters that I've seen before include "Fleshy bivalves, may yield pearls" , "Succulent bivalves, perhaps with pearls" , "Rosy set of odd shellfish" , "Marine molluscs" , "Sources of pearls" .)

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

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