Blow the whistle (4)
I believe the answer is:
toot
'blow the whistle' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for toot that I've seen before include "Noise made by car horn" , "Sound one's horn" , "Blow a horn" , "Short sounding of horn" , "Do it on a flute" .)