Atmospheric pollution (4)
I believe the answer is:
smog
'atmospheric pollution' 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 "Bad visibility caused by emissions from chimneys" , "Smokey fog" , "Industrially polluted air" , "Dirty mixture of smoke and fog" , "Fog and smoke mix" .)