So go back to the monster (4)
I believe the answer is:
ogre
'the monster' is the definition.
(fairy tale monster)
'so go back' is the wordplay.
'so' becomes 'ergo' (synonyms).
'go back' is a reversal indicator.
'ergo' back-to-front is 'OGRE'.
'to' is the link.
(Other definitions for ogre that I've seen before include "Giant: monster" , "Has a monstrous role" , "Giant of folklore who ate humans" , "Wicked giant" , "character in 'Puss in Boots'" .)