Monster therefore keeled over (4)
I believe the answer is:
ogre
'monster' is the definition.
(fairy tale monster)
'therefore keeled over' is the wordplay.
'therefore' becomes 'ergo' (I've seen this in another clue).
'keeled over' says the letters should be written in reverse.
'ergo' reversed gives 'OGRE'.
(Other definitions for ogre that I've seen before include "The giant is some goer" , "Ergo (anag.)" , "Terrifying man" , "'Demon, fiend (4)'" , "Monstrous giant" .)