I am overturning anger - it's not real (6)
I believe the answer is:
mirage
'it's not real' is the definition.
(I've seen this in another clue)
'i am overturning anger' is the wordplay.
'i am' becomes 'I'm' (contraction).
'overturning' says the letters should be written backwards.
'anger' becomes 'rage' (rage is a kind of anger).
'im' back-to-front is 'mi'.
'mi'+'rage'='MIRAGE'
(Other definitions for mirage that I've seen before include "Something illusory and unattainable" , "French flier" , "fighter craft" , "Illusions in the desert" , "Optical illusion, in a desert say" .)