Marks vehicles on style, at first (5)
I believe the answer is:
scars
'marks' is the definition.
('scar' can be a synonym of 'mark')
'vehicles on style at first' is the wordplay.
'vehicles' becomes 'cars' (car is a kind of vehicle).
'on' means one lot of letters go next to another.
'style at first' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'cars' put after 's' is 'SCARS'.
Can you help me to learn more?
(Another definition for scars that I've seen is " Marks left by wounds".)