One who's on your side (8)
'one who's on your side' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ally that I've seen before include "He fights on your side" , "Associate that provides assistance" , "Colleague, confederate" , "Friend in time of need" , "Join by marriage, treaty etc" .)
(Another definition for teammate that I've seen is " fellow player".)