Terrible being so upset (4)
I believe the answer is:
ogre
'terrible being' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'so upset' is the wordplay.
'so' becomes 'ergo' (synonyms).
'upset' says the letters should be written backwards.
'ergo' in reverse letter order is 'OGRE'.
Can you help me to learn more?
(Other definitions for ogre that I've seen before include "'Demon, fiend (4)'" , "Has a monstrous role" , "Terrifying giant" , "Wicked and inhuman person" , "ugly one" .)