Animal's posterior (4)
I believe the answer is:
hind
I believe this clue is a double definition.
'animal's' is the first definition.
(I've seen this before)
'posterior' is the second definition.
('hind' can be similar in meaning to 'posterior')
(Other definitions for hind that I've seen before include "Be in front of this farm-worker" , "Female red deer" , "Rear - animal" , "posterior" , "Rear - female deer" .)