Man-eating monster in fairy tale therefore repelled (4)
I believe the answer is:
ogre
'man-eating monster in fairy tale' is the definition.
'therefore repelled' is the wordplay.
'therefore' becomes 'ergo' (I've seen this before).
'repelled' 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 "Cruel and wicked person" , "Wicked and inhuman person" , "Giant of folklore who liked to eat humans" , "Giant of folklore who ate humans" , "Monster, troll" .)