Unhealthy vapour (6)
I believe the answer is:
miasma
'unhealthy vapour' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for miasma that I've seen before include "Unwholesome or foreboding atmosphere" , "foul atmosphere" , "Oppressive atmosphere" , "In poor atmospheric conditions" , "Foul vapours or unwholesome air" .)