Sing with lips closed (3)
I believe the answer is:
hum
'sing with lips 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 "Unpleasant smell" , "Sound of a bee" , "Make musical sound with closed lips" , "Stench" , "Low sound of bad smell?" .)