Giant, one bearing down on a queen (7)
I believe the answer is:
titania
'queen' is the definition.
(fairy queen)
'giant one bearing down on a' is the wordplay.
'giant' becomes 'titan' (titans are giant mythological creatures).
'one' becomes 'i' (Roman numeral).
'bearing down on' means one lot of letters go next to another.
'titan'+'i'+'a'='TITANIA'
(Other definitions for titania that I've seen before include "Queen of the fairies - Uranus satellite" , "Queen of fairyland" , "asinine lover" , "Bottom attracts me" , "in the D [DREAM] ..." .)