Treasure Margaret returned (3)
I believe the answer is:
gem
'treasure' is the definition.
(I know that treasure can be written as gem)
'margaret returned' is the wordplay.
'margaret' becomes 'Meg' (short for Margaret).
'returned' is a reversal indicator.
'meg' back-to-front is 'GEM'.
(Other definitions for gem that I've seen before include "one of the Cairngorms?" , "Treasured person" , "Setter might bed this" , "A jewel or precious stone" , "Valuable stone" .)