Therefore, 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' backwards is 'ERGO'.
(Other definitions for ergo that I've seen before include "Therefore - what a goer" , "'Therefore, to the old Romans (4)'" , "so philosopher wrote in Latin" , "Consequently" , "'Cogito, ---- sum'" .)