Moo (3)
I believe the answer is:
low
'moo' is the definition.
(I know that moo can be written as low)
This is the entire clue.
(Other definitions for low that I've seen before include "Inferior; moo" , "sound from Hereford?" , "Not high or loud" , "Moo; not high" , "Unimportant sort of owl" .)