An ally in one form or another (9)
I believe the answer is:
classmate
'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).
'class'+'mate'='CLASSMATE'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for classmate that I've seen before include "School associate" , "one of the others being in form" , "Schoolfellow" , "Fellow pupil" , "Fellow student" .)