Humming sound (5)
I believe the answer is:
drone
'humming sound' is the definition.
(I've seen this in another clue)
This is all the clue.
(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" .)