Sound the horn (4)
I believe the answer is:
'sound the horn' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for honk that I've seen before include "Toot one's horn" , "Cry of the wild goose" , "Cry of a goose - sound of a horn" , "Toot of a horn - cry of a goose - smell nasty" .)