Sing with your mouth closed (3)
I believe the answer is:
hum
'sing with your mouth closed' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for hum that I've seen before include "Sound of busy activity" , "Unpleasant smell" , "Sing with closed lips" , "Buzz; stink" , "Smell badly (slang)" .)