Marine bivalve (6)
I believe the answer is:
'marine bivalve' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for oyster that I've seen before include "Bivalve that may nurture a pearl" , "Succulent bivalve, may bear a pearl" , "Pearl-producing bivalve" , "Food shellfish" , "Type of mollusc" .)