Small cat producing air pollution (4)
I believe the answer is:
smog
'air pollution' is the definition.
(smog is a kind of air pollution)
'small cat' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cat' becomes 'mog' (I've seen this before**).
's'+'mog'='SMOG'
'producing' acts as a link.
(Other definitions for smog that I've seen before include "Visible air pollution" , "Bad visibility caused by emissions from chimneys" , "Fog and smoke mix" , "Smoky fog" , "Polluted haze" .)