Therefore the monster returns (4)
I believe the answer is:
ergo
'therefore' is the definition.
(I've seen this before)
'monster returns' is the wordplay.
'monster' becomes 'ogre' (fairy tale monster).
'returns' shows that the letters should be reversed in order.
'ogre' in reverse letter order is 'ERGO'.
'the' acts as a link.
(Other definitions for ergo that I've seen before include "so philosopher wrote in Latin" , "Therefore - what a goer" , "Hence" , "Therefore (Latin)" , "It follows" .)