Monster therefore returns (4)
I believe the answer is:
ogre
'monster' is the definition.
('ogre' can be a synonym of 'monster')
'therefore returns' is the wordplay.
'therefore' becomes 'ergo' (I've seen this before).
'returns' shows that the letters should be reversed in order.
'ergo' reversed gives 'OGRE'.
(Other definitions for ogre that I've seen before include "bad-tempered person" , "terrible person" , "Ergo (anag.)" , "Terrifying monster" , "Fairy tale giant" .)