Optical illusion caused by atmospheric conditions (6)
I believe the answer is:
mirage
'optical illusion caused by atmospheric conditions' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mirage that I've seen before include "It doesn't exist" , "Optical illusion caused by atmospheric conditions" , "See it to believe it?" , "fantasy" , "vision" .)