Grim villain, therefore overturned (4)
I believe the answer is:
ogre
'grim villain' is the definition.
'ogre' can be an answer for 'villain' (I've seen this in another clue). I am not certain of the 'grim' bit.
'therefore overturned' is the wordplay.
'therefore' becomes 'ergo' (I've seen this in another clue).
'overturned' is a reversal indicator.
'ergo' in reverse letter order is 'OGRE'.
Can you help me to learn more?
(Other definitions for ogre that I've seen before include "Monstrous creature" , "Cruel, fearsome person" , "Cruel giant of folklore" , "The giant is some goer" , "terrible person" .)