Fellow has to train monkey (8)
I believe the answer is:
mandrill
'monkey' is the definition.
(mandrill is a kind of monkey)
'fellow has to train' is the wordplay.
'fellow' becomes 'man' (I've seen this before).
'has' means one lot of letters go next to another.
'to 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" , "Ape" , "West African baboon" , "Large baboon" , "Colourful baboon in the damn rill" .)