A low sound (3)
I believe the answer is:
moo
I believe this clue is a double definition.
'a low' is the first definition.
(both can mean to make the noise of a cow)
'sound' is the second definition.
(moo is a kind of sound)
(Other definitions for moo that I've seen before include "Guernsey noise?" , "Low (of cattle)" , "Cartoon sound of a cow" , "Cow's sound" , "unpopular woman" .)