Monster thus turned up (4)
I believe the answer is:
ogre
'monster' is the definition.
(fairy tale monster)
'thus turned up' is the wordplay.
'thus' becomes 'ergo' (I've seen this before).
'turned up' says the letters should be written backwards.
'ergo' backwards is 'OGRE'.
(Other definitions for ogre that I've seen before include "Bogeyman" , "Monstrous person" , "stern one who inspires fear" , "Cruel giant of folklore" , "Fairy tale giant" .)