Bumper sticker verb (5)
I believe the answer is:
brake
'bumper sticker verb' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for brake that I've seen before include "Thicket; estate car" , "Car stopper or thicket" , "Slower" , "Decelerate" , "Slowing-down device" .)