Air pollution (4)
I believe the answer is:
smog
'air pollution' is the definition.
(I know that smog is a type of air pollution)
This is the entire clue.
(Other definitions for smog that I've seen before include "Fog polluted by smoke" , "Polluted haze" , "Bad visibility caused by emissions from chimneys" , "Fog and smoke mix" , "Dirty mixture of smoke and fog" .)