Gem fairy's presented to Dorothy (7)
I believe the answer is:
peridot
'gem' is the definition.
(peridot is a kind of gem)
'fairy's presented to dorothy' is the wordplay.
'fairy' becomes 'peri' (Persian mythological being).
'presented to' says to put letters next to each other.
'dorothy' becomes 'dot' (nickname for someone called Dorothy).
'peri'+'dot'='PERIDOT'
(Other definitions for peridot that I've seen before include "Top ride (anag) - gemstone" , "olivine" , "A green semi-precious stone" , "I deport (anag) - gemstone" , "Pale green gemstone" .)