Optical illusion (6)
I believe the answer is:
mirage
'optical illusion' is the definition.
(mirage is a kind of optical illusion)
This is all the clue.
(Other definitions for mirage that I've seen before include "image, not what it seems" , "Hallucination caused by hot air" , "Deceptive image, in the desert say" , "Optical illusion, in a desert say" , "Illusory image" .)