Smoky fog (4)
I believe the answer is:
smog
'smoky fog' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for smog that I've seen before include "Pollution haze/fog" , "Pea-souper" , "Dirty mixture of smoke and fog" , "Pollutant haze" , "Polluted haze" .)