Fog (4)
'fog' is the definition.
(I have seen 'Smoky fog' mean 'smog' so perhaps 'fog' could also mean 'smog')
This is all the clue.
(Other definitions for smog that I've seen before include "Smoky haze" , "Fog polluted by smoke" , "Pea-souper" , "Pollutant haze" , "Thick polluted mist" .)
'fog' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for mist that I've seen before include "Watery film" , "Fine spray" , "Watery vapour" , "Cloud; thin fog" , "Mizzle; light rain" .)