Races including contest chasing American cars? (9)

'cars?' is the definition.
(runabout is a kind of car)

'races including contest chasing american' is the wordplay.
'races' becomes 'runs' ('run' can be a synonym of 'race').
'including' means one lot of letters goes inside another.
'contest' becomes 'bout' (boxing contest).
'chasing' says to put letters next to each other (some letters chase after others).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'bout' put after 'a' is 'about'.
'runs' going around 'about' is 'RUNABOUTS'.