An ally in one form or another (9)
I believe the answer is:
Here is my best explanation:
'an ally' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'one form or another' is the wordplay.
'one form' becomes 'class' (synonyms).
'or another' becomes 'mate' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' acts as a link.
(Other definitions for classmate that I've seen before include "School associate", "one of the others being in form", "someone from school?", "Fellow pupil", "One in same form", "Fellow student" and "Schoolfellow".)