Polluted atmosphere created by small cat (4)
I believe the answer is:
smog
'polluted atmosphere' is the definition.
(I've seen this before)
'by small cat' is the wordplay.
'by' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cat' becomes 'mog' (informal British term).
's' next to 'mog' is 'SMOG'.
'created' acts as a link.
(Other definitions for smog that I've seen before include "Polluted haze" , "Pollution fog" , "Bad visibility caused by emissions from chimneys" , "Smoky fog" , "Pollutant haze" .)