Air apparent? (4)
I believe the answer is:
smog
'air apparent?' 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 "Man-made atmospheric pollution" , "Fog polluted by smoke" , "Polluted haze" , "Pollutant haze" , "Pollutant fog" .)