Motorway activity rolling past stokes air pollution (4)

I believe the answer is:

smog

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in The Sunday Times and The Times.

Want a hint initially instead of a full solution? Install my app