Staff train monkey (8)
I believe the answer is:
mandrill
'monkey' is the definition.
(I know that Mandrill is a type of baboon)
'staff train' is the wordplay.
'staff' becomes 'man' (as in staffing or manning a particular place).
'train' becomes 'drill' (drilling is a kind of training).
'man'+'drill'='MANDRILL'
(Other definitions for mandrill that I've seen before include "Baboon with colourful face" , "African baboon with red and blue muzzle and hindquarters" , "Colourful baboon in the damn rill" , "Monkey with red and blue hindquarters" , "Ape" .)