Small cat polluted air (4)
I believe the answer is:
smog
'polluted air' 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'
(Other definitions for smog that I've seen before include "Pollution haze/fog" , "Fog and smoke mix" , "Pollution fog" , "Bad visibility caused by emissions from chimneys" , "Polluted atmosphere" .)