Motorway activity rolling past stokes air pollution (4)
I believe the answer is:
smog
'air pollution' is the definition.
(I know that smog is a type of air pollution)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for smog that I've seen before include "Pollution haze/fog" , "Fog and smoke mix" , "Pollution fog" , "Polluted haze" , "Smoky fog" .)