Source of milk (5)
I believe the answer is:
udder
'source of milk' is the definition.
(eg a cow's udder)
This is all the clue.
(Other definitions for udder that I've seen before include "Animal mammary gland" , "Animal organ with more than one teat" , "Milk supply" , "Cow's teat" , "Mammary gland of cow" .)