Polluted atmosphere originally stifling cat (4)
I believe the answer is:
smog
'polluted atmosphere' is the definition.
(I've seen this before)
'originally stifling cat' is the wordplay.
'originally' suggests taking the first letters.
'cat' becomes 'mog' (informal British term).
The first letter of 'stifling' is 's'.
's'+'mog'='SMOG'
(Other definitions for smog that I've seen before include "Fog polluted by smoke" , "Smokey fog" , "Pollution haze/fog" , "Pollutant fog" , "Smoky fog" .)