Foster's back (4)
I believe the answer is:
rear
I believe this clue is a double definition.
'foster's' is the first definition.
(both can mean to bring up or raise)
'back' is the second definition.
(something's rear is its back)
(Other definitions for rear that I've seen before include "Bring up - back end" , "early, long before now" , "Buttocks - rise up" , "Of horse, rise up on hind legs" , "Bring up - the tail" .)