Thin mist (4)
I believe the answer is:
haze
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">thin mist</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for haze that I've seen before include "Film" , "Lack of definition or precision" , "A thin mist, perhaps from heat" , "Vapour" , "Daze, fog" .)