Another one who is in form given marks? (9)
I believe the answer is:
classmate
'another one who' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'form given marks?' is the wordplay.
'form' becomes 'class' (synonyms).
'given marks?' becomes 'mate' (I can't justify this - if you can you should give a lot more credence to this answer).
'class'+'mate'='CLASSMATE'
'is in' is the link.
(Other definitions for classmate that I've seen before include "Fellow pupil" , "someone from school?" , "one of the others being in form" , "Schoolfellow" , "School associate" .)