Gemstone returned to dreadful party leader (7)
I believe the answer is:
peridot
'gemstone' is the definition.
(peridot is a kind of gemstone)
'returned to dreadful party leader' is the wordplay.
'returned' shows that the letters should be reversed in order.
'dreadful' becomes 'dire' (I've seen this before).
'party leader' becomes 'p' (I've seen this in other clues).
'to'+'dire'+'p'='todirep'
'todirep' in reverse letter order is 'PERIDOT'.
(Other definitions for peridot that I've seen before include "Top ride (anag) - gemstone" , "Pale green gemstone" , "I deport (anag) - gemstone" , "A green semi-precious stone" , "mineral" .)