Fairy tale's opening describing party is a gem (7)
I believe the answer is:
peridot
'a gem' is the definition.
(peridot is a kind of gem)
'fairy tale's opening describing party' is the wordplay.
'fairy' becomes 'peri' (Persian mythological being).
'opening' says to take the initial letters.
'describing' indicates putting letters inside (describe can mean to form the outline of).
'party' becomes 'do' (do is a kind of party).
The first letter of 'tales' is 't'.
'peri'+'t'='perit'
'perit' going around 'do' is 'PERIDOT'.
'is' is the link.
(Other definitions for peridot that I've seen before include "olivine" , "A green semi-precious stone" , "I deport (anag) - gemstone" , "Gem" , "Top ride (anag) - gemstone" .)