Humming sound (5)
I believe the answer is:
drone
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">humming sound</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 drone that I've seen before include "Continuous low-pitched note" , "Pilotless, radio-controlled aircraft" , "mollusc" , "Talk boringly; male bee" , "Idler or male honey-bee" .)