Unhealthy atmosphere (6)
I believe the answer is:
miasma
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unhealthy atmosphere</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for miasma that I've seen before include "Unpleasant atmosphere" , "Foul vapour" , "Unpleasant smell/vapour" , "Bad air" , "Unpleasant vapour or influence" .)