Deer's rear (4)
I believe the answer is:
hind
I believe this clue is a double definition.
'deer's' is the first definition.
(hind is a kind of deer)
'rear' is the second definition.
('hind' can be a synonym of 'rear')
(Other definitions for hind that I've seen before include "Be much better than (someone)" , "Female beast" , "Be in front of this farm-worker" , "Female red deer" , "Rear - animal" .)