Sound the horn going forward and reversing (4)
I believe the answer is:
toot
I believe this clue is a double definition.
'sound the horn' is the first definition.
(I've seen this before)
'going forward and reversing' is the second definition.
(Other definitions for toot that I've seen before include "Brief blast on a horn" , "Briefly blow" , "Use horn" , "Short sound of horn" , "Short sounding of horn" .)