Unpleasant atmosphere (6)
I believe the answer is:
miasma
'unpleasant atmosphere' is the definition.
(miasma is an unpleasant or noxious vapour)
This is all the clue.
(Other definitions for miasma that I've seen before include "In poor atmospheric conditions" , "Bad atmosphere" , "Unpleasant, polluted, air" , "An unpleasant or unhealthy smell or vapour" , "Foul vapour" .)