Unwholesome atmosphere (6)
I believe the answer is:
miasma
'unwholesome atmosphere' is the definition.
(miasma can mean an unpleasant atmosphere)
This is the entire clue.
(Other definitions for miasma that I've seen before include "Noxious vapour" , "Bad atmosphere" , "Unwholesome atmosphere, unhealthy vapors" , "Effluvium" , "Stench" .)