Margaret shortly turned to stone (3)
I believe the answer is:
gem
'stone' is the definition.
(a gemstone)
'margaret shortly turned' is the wordplay.
'margaret shortly' becomes 'Meg' (short for Margaret; 'shortly' suggests a short form).
'turned' says the letters should be written in reverse.
'meg' reversed gives 'GEM'.
'to' is the link.
(Other definitions for gem that I've seen before include "A precious stone like a diamond or emerald etc" , "Precious or semi-precious stone" , "perfect specimen" , "Emerald or sapphire, say" , "36 Across or any other jewel" .)