Slow (a vehicle) (5)
I believe the answer is:
brake
'slow a vehicle' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for brake that I've seen before include "Decelerate" , "Cause to slow down" , "Device for slowing or stopping car" , "Device for slowing vehicle" , "Slowing-down device" .)