Discolour a small vehicle (4)
I believe the answer is:
scar
'discolour' is the definition.
Although both the answer and definition are verbs in their base form, I can't see how one could define the other.
'a small vehicle' is the wordplay.
'a small' becomes 's' (abbreviation - e.g. clothes size).
'vehicle' becomes 'car' (car is a kind of vehicle).
's'+'car'='SCAR'
Can you help me to learn more?
(Other definitions for scar that I've seen before include "Mark from a wound left on the skin" , "Mark left by healed wound" , "Bare craggy rock" , "area of wound" , "Mark of a wound remaining on the skin" .)