Skyline obscurer (4)
I believe the answer is:
haze
'skyline obscurer' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for haze that I've seen before include "A thin mist, perhaps from heat" , "Mistiness" , "Vapour" , "Mist or shimmer due to heat" , "'Mist, light fog (4)'" .)