Decelerated (6)
(Other definitions for braked that I've seen before include "slowed down" , "Brought car to a halt" , "Halted" .)
'decelerated' is the definition.
(slowing is a kind of decelerating)
This is the entire clue.
(Other definitions for slowed that I've seen before include "Braked" , "Applied the brakes" , "Reduced speed" , "Decelerated" .)