Go around on southern motorway creating air pollution (4)
I believe the answer is:
smog
'air pollution' is the definition.
(smog is a kind of air pollution)
'go around on southern motorway' is the wordplay.
'around' says the letters should be written backwards.
'on' says to put letters next to each other.
'southern' becomes 's' (abbreviation).
'motorway' becomes 'm'.
'go' written backwards gives 'og'.
's'+'m'='sm'
'og' after 'sm' is 'SMOG'.
'creating' is the link.
(Other definitions for smog that I've seen before include "Pollutant fog" , "Man-made atmospheric pollution" , "Thick polluted mist" , "Pollution haze/fog" , "Smoky haze" .)