Giant of giants, really enormous initially (4)
I believe the answer is:
ogre
'giant' is the definition.
(I know that ogre is a type of giant)
'of giants really enormous initially' is the wordplay.
'initially' says to take the initial letters.
The first letters of 'of giants really enormous' is 'OGRE'.
(Other definitions for ogre that I've seen before include "Monstrous person" , "Terrifying person" , "Fairy tale giant" , "Man-eating monster of fairy tales" , "Ugly giant" .)