Foster's back (4)
I believe the answer is:
rear
I believe this is a double definition.
'foster's' is the first definition.
(I've seen this before)
'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" .)