Noxious atmosphere (6)
I believe the answer is:
miasma
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">noxious atmosphere</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 "Effluvium" , "Oppressive atmosphere" , "Accomplished" , "An unpleasant or unhealthy smell or vapour" , "Foul vapour" .)