Primate is partisan when speaking (7)
I believe the answer is:
gorilla
'primate' is the definition.
(I know that Gorilla is a type of primate)
'partisan when speaking' is the wordplay.
'partisan' becomes 'guerrilla' (similar in meaning).
'when speaking' shows a homophone (sound like).
'guerrilla' is a homophone of 'GORILLA'.
'is' acts as a link.
(Other definitions for gorilla that I've seen before include "Primate - sturdily-built aggressive man" , "Ape - aggressive muscleman" , "thug" , "big mammal" , "Large chest-beating ape" .)