Unhealthy atmosphere (6)
I believe the answer is:
miasma
'unhealthy 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 "Unpleasant atmosphere" , "Foul vapour" , "Unpleasant smell/vapour" , "Bad air" , "Unpleasant vapour or influence" .)