Optical illusion caused by atmospheric conditions (6)
I believe the answer is:
mirage
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">optical illusion caused by atmospheric conditions</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)