For that reason monster returns (4)
I believe the answer is:
ergo
'for that reason' is the definition.
(I've seen this in another clue)
'monster returns' is the wordplay.
'monster' becomes 'ogre' (fairy tale monster).
'returns' shows that the letters should be reversed in order.
'ogre' backwards is 'ERGO'.
(Other definitions for ergo that I've seen before include "Consequently" , "It follows" , "Therefore - what a goer" , "'Therefore, to the old Romans (4)'" , "Therefore (Latin)" .)