Therefore a monstrous giant retires (4)
I believe the answer is:
ergo
'therefore' is the definition.
(I've seen this before)
'a monstrous giant retires' is the wordplay.
'a monstrous giant' becomes 'ogre' (I've seen this before).
'retires' says the letters should be written in reverse.
'ogre' backwards is 'ERGO'.
(Other definitions for ergo that I've seen before include "so philosopher wrote in Latin" , "Consequently" , "Hence" , "Therefore (Latin)" , "Thus" .)