Monstrous giant (4)
I believe the answer is:
ogre
'monstrous giant' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ogre that I've seen before include "Monstrous person" , "Cruel giant of folklore" , "Ugly giant" , "Giant who eats people" , "Man-eating monster of fairy tales" .)