Backward - hence a monster (4)
I believe the answer is:
ogre
'a monster' is the definition.
(fairy tale monster)
'backward hence' is the wordplay.
'backward' shows that the letters should be reversed in order.
'hence' becomes 'ergo' (I've seen this in another clue).
'ergo' backwards is 'OGRE'.
(Other definitions for ogre that I've seen before include "stern one who inspires fear" , "Wicked giant" , "Ugly giant" , "Giant: monster" , "Grim(m) beast?" .)