Bring up small posteriors (5)
I believe the answer is:
rears
'posteriors' is the definition.
('rear' can be a synonym of 'posterior')
'bring up small' is the wordplay.
'bring up' becomes 'rear' (as in rearing children).
'small' becomes 's' (abbreviation - e.g. clothes size).
'rear'+'s'='REARS'
(Other definitions for rears that I've seen before include "Back ends" , "Fosters, brings up" , "bucks" , "Sets up" , "Raises" .)