Noxious atmosphere (6)
I believe the answer is:
miasma
'noxious atmosphere' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for miasma that I've seen before include "Effluvium" , "Oppressive atmosphere" , "Accomplished" , "An unpleasant or unhealthy smell or vapour" , "Foul vapour" .)