Gem from an oyster (5)
I believe the answer is:
pearl
'gem from an oyster' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pearl that I've seen before include "White - jewel" , "Off-white colour" , "Paragon" , "Precious stone from shellfish" , "one regularly in stitches" .)