Meg returned diamond (3)
I believe the answer is:
gem
'diamond' is the definition.
(I've seen this in other clues)
'meg returned' is the wordplay.
'returned' says the letters should be written backwards.
'meg' back-to-front is 'GEM'.
(Other definitions for gem that I've seen before include "A precious stone like a diamond or emerald etc" , "Emerald or sapphire, say" , "Valuable stone" , "Pearl" , "Valued person; jewel" .)