Primate replaces the ushers (6)
I believe the answer is:
rhesus
'primate' is the definition.
(rhesus is a kind of primate)
'replaces the ushers' is the wordplay.
'replaces' indicates anagramming the letters (I've seen 'replaced' mean this).
'ushers' anagrammed gives 'RHESUS'.
(Other definitions for rhesus that I've seen before include "Kind of monkey or blood group" , "Small Indian monkey" , "(Kind of monkey) used in medical experiments" , "King, in Greek mythology" , "Rushes (anag.)" .)