Sound of cow that's depressed (3)
I believe the answer is:
low
This clue is a double definition.
'sound of cow' is the first definition.
(low can mean a mooing sound)
'depressed' is the second definition.
(synonyms)
'that's' is the link.
(Other definitions for low that I've seen before include "Inferior, cow's call" , "Deep; moo" , "Not high - that's mean" , "Inferior; not loud" , "Dejected; moo" .)