Mother's barrier? (3)
I believe the answer is:
dam
This clue is a double definition.
'mother's' is the first definition.
(female parent of an animal)
'barrier?' is the second definition.
(dam is a kind of barrier)
(Other definitions for dam that I've seen before include "River restraint" , "Female parent; obstruction" , "Weir" , "Reservoir barrier" , "Wall restraining water" .)