Polluted air (4)
I believe the answer is:
smog
'polluted air' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for smog that I've seen before include "Pollutant fog" , "Industrially polluted air" , "Pollution haze/fog" , "Dirty mixture of smoke and fog" , "Pollution fog" .)