Pollution (4)
I believe the answer is:
smog
'pollution' is the definition.
(I know that smog is a type of air pollution)
This is all the clue.
(Other definitions for smog that I've seen before include "Pea-souper" , "Polluted atmosphere" , "Smokey fog" , "Pollution haze/fog" , "Fog and smoke mix" .)