Unhealthy vapour (6)
I believe the answer is:
miasma
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unhealthy vapour</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 miasma that I've seen before include "Unwholesome or foreboding atmosphere" , "foul atmosphere" , "Oppressive atmosphere" , "In poor atmospheric conditions" , "Foul vapours or unwholesome air" .)