Stone Margaret's turned over (3)
I believe the answer is:
gem
'stone' is the definition.
(thesaurus)
'margaret's turned over' is the wordplay.
'margaret' becomes 'meg'.
'turned over' says the letters should be written in reverse.
'meg' reversed gives 'GEM'.
(Other definitions for gem that I've seen before include "Half of 7 [SIGN OF THE ZODIAC]" , "A precious stone like a diamond or emerald etc" , "Setter might bed this" , "Valuable stone" , "Pearl" .)