Blow the whistle (4)
I believe the answer is:
toot
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">blow the whistle</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)