Margaret turns up with the treasure (3)
I believe the answer is:
gem
'the treasure' is the definition.
(gem is a kind of treasure)
'margaret turns up' is the wordplay.
'margaret' becomes 'meg'.
'turns up' says the letters should be written backwards.
'meg' reversed gives 'GEM'.
'with' acts as a link.
(Other definitions for gem that I've seen before include "Diamond, ruby, or emerald" , "A jewel of an Orangeman" , "Setter might bed this" , "Valuable stone" , "Treasured person" .)