Small cat in urban haze? (4)
I believe the answer is:
smog
'urban haze' is the definition.
(I've seen this in another clue)
'small cat' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cat' becomes 'mog' (informal British term).
's'+'mog'='SMOG'
'in' is the link.
(Other definitions for smog that I've seen before include "Smokey fog" , "Polluted atmosphere" , "Polluted haze" , "Pea-souper" , "Bad visibility caused by emissions from chimneys" .)