Oppressive atmosphere (6)
I believe the answer is:
miasma
'oppressive atmosphere' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for miasma that I've seen before include "Stench" , "Stink" , "Foul vapour" , "Oppressive atmosphere" , "Unwholesome atmosphere, unhealthy vapors" .)