Fairy ultimately positioned over top of tree? It's a gem! (7)
I believe the answer is:
peridot
'a gem' is the definition.
(peridot is a kind of gem)
'fairy ultimately positioned over top of tree?' is the wordplay.
'fairy' becomes 'peri' (Persian mythological being).
'ultimately' indicates one should take the final letters.
'over' becomes 'o' (cricket abbreviation**).
'top of' says to take the initial letters.
The last letter of 'positioned' is 'd'.
The first letter of 'tree' is 't'.
'peri'+'d'+'o'+'t'='PERIDOT'
'it's' acts as a link.
(Other definitions for peridot that I've seen before include "Gem" , "Pale green gemstone" , "I deport (anag) - gemstone" , "A green semi-precious stone" , "olivine" .)