Dirty air (4)
I believe the answer is:
'dirty air' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for smog that I've seen before include "Thick polluted mist" , "Pollutant haze" , "Fog and smoke mix" , "Fog polluted by smoke" , "Industrially polluted air" .)