Sound a horn (4)
I believe the answer is:
toot
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sound a horn</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for toot that I've seen before include "Blow a horn" , "Sound one's horn" , "Briefly blow" , "Noise made by car horn" , "Horn's sound (T, not H)" .)