Warning hooter (6)
I believe the answer is:
klaxon
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">warning hooter</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for klaxon that I've seen before include "Warning hooter" , "source of warning" , "warning device" , "Old car horn" , "Warning horn" .)