Red team's in reversing vehicle (7)
I believe the answer is:
marxist
'red' is the definition.
(red can mean a communist)
'team's in reversing vehicle' is the wordplay.
'team's' becomes 'XI's' (a football team, from Roman numerals for tt).
'in' indicates putting letters inside.
'reversing' says the letters should be written in reverse.
'vehicle' becomes 'tram' (tram is a kind of vehicle).
'tram' backwards is 'mart'.
'xis' going into 'mart' is 'MARXIST'.
(Other definitions for marxist that I've seen before include "A leftie once" , "left-winger" , "Revolutionary" , "left-wing" , "Red" .)