Giant one found with a fairy queen (7)
I believe the answer is:
titania
'fairy queen' is the definition.
(I've seen this before)
'giant one found with a' is the wordplay.
'giant' becomes 'titan' (titans are giant mythological creatures).
'one' becomes 'i' (Roman numeral).
'found with' means one lot of letters go next to another.
'titan'+'i'+'a'='TITANIA'
(Other definitions for titania that I've seen before include "Wife of Oberon (MND)" , "asinine lover" , "Shakespeare's queen of the fairies" , "Fairy queen (AMND)" , "Oberon's better half?" .)