Mammary gland part (4)
I believe the answer is:
teat
'mammary gland part' is the definition.
('teat' is part of 'mammary gland')
This is all the clue.
(Other definitions for teat that I've seen before include "Part of feeding bottle" , "Bulb on infant's bottle" , "Piece on baby's bottle" , "essential for babies" , "Nipple or dug" .)