Thick polluted mist (4)
I believe the answer is:
smog
'thick polluted mist' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for smog that I've seen before include "Pollution fog" , "Dirty mixture of smoke and fog" , "Pollutant fog" , "Pollutant haze" , "Smoky fog" .)