Skyline haze (4)
I believe the answer is:
smog
'skyline haze' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for smog that I've seen before include "Bad visibility caused by emissions from chimneys" , "Thick polluted mist" , "Man-made atmospheric pollution" , "Dirty mixture of smoke and fog" , "Visible air pollution" .)