Cow's mammary organ (5)
I believe the answer is:
udder
'cow's mammary organ' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for udder that I've seen before include "Natural container for milk?" , "Cow's teat" , "Cow's milk source" , "Teat of a cow" , "Milk producing gland of cows etc" .)