Mark left by injury to head of security by vehicle (4)
I believe the answer is:
scar
'mark left by injury' is the definition.
(I've seen this before)
'head of security by vehicle' is the wordplay.
'head of' says to take the initial letters.
'by' is a charade indicator (letters next to each other).
'vehicle' becomes 'car' (car is a kind of vehicle).
The initial letter of 'security' is 's'.
's'+'car'='SCAR'
'to' acts as a link.
(Other definitions for scar that I've seen before include "Injury mark - cliff" , "Evidence of an old cut?" , "Sign of wound" , "Identifying mark" , "Fish" .)