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' shows that the letters should be reversed in order.
'ogre' written backwards gives 'ERGO'.
(Other definitions for ergo that I've seen before include "so philosopher wrote in Latin" , "Consequently" , "Hence" , "Therefore (Latin)" , "Thus" .)