Call fighter an ape (7)
I believe the answer is:
gorilla
'an ape' is the definition.
(I know that gorilla is a type of great ape)
'call fighter' is the wordplay.
'call' indicates a 'sounds like' (homophone) clue.
'fighter' becomes 'guerrilla' (as in guerrilla warfare).
'guerrilla' is a homophone of 'GORILLA'.
(Other definitions for gorilla that I've seen before include "Big ape" , "Aggressive-looking type" , "The biggest ape" , "big mammal" , "African great ape" .)