Oyster product (5)
I believe the answer is:
pearl
'oyster product' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for pearl that I've seen before include "Gem from oyster" , "Lustrous structure inside an oyster-shell" , "Lustrous globule prized as a gem" , "Drop" , "Gem from a bivalve" .)