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