I'm taken aback by anger? That's an illusion (6)
I believe the answer is:
mirage
'that's an illusion' is the definition.
(I've seen this before)
'i'm taken aback by anger?' is the wordplay.
'taken aback' says the letters should be written in reverse.
'by' says to put letters next to each other.
'anger?' becomes 'rage' (rage is a kind of anger).
'im' backwards is 'mi'.
'mi'+'rage'='MIRAGE'
(Other definitions for mirage that I've seen before include "French flier" , "image, not what it seems" , "You might see this in desert" , "fighter craft" , "Deceptive image, in the desert say" .)