Large monkey (6)
'large monkey' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for baboon that I've seen before include "Ground-living primate" , "Large primate" , "Stupid person" , "Large monkey with a doglike muzzle" , "animal" .)
(Other definitions for mandrill that I've seen before include "West African baboon-like primate" , "African baboon with red and blue muzzle and hindquarters" , "Monkey with red and blue hindquarters" , "Ape" , "Colourful baboon in the damn rill" .)