Monster's so contrary (4)
I believe the answer is:
ogre
'monster's' is the definition.
(fairy tale monster)
'so contrary' is the wordplay.
'so' becomes 'ergo' (synonyms).
'contrary' shows that the letters should be reversed in order.
'ergo' in reverse letter order is 'OGRE'.
(Other definitions for ogre that I've seen before include "Has a monstrous role" , "Giant of folklore who liked to eat humans" , "Giant who eats people" , "Cruel, fearsome person" , "Terrifying giant" .)