Monster therefore coming back (4)
I believe the answer is:
ogre
'monster' is the definition.
(synonyms)
'therefore coming back' is the wordplay.
'therefore' becomes 'ergo' (I've seen this before).
'coming back' says the letters should be written in reverse.
'ergo' back-to-front is 'OGRE'.
(Other definitions for ogre that I've seen before include "Human-eating giant" , "Giant: monster" , "Bogeyman" , "Terrifying monster" , "man-eater" .)