Final check on form for an acquaintance (9)
I believe the answer is:
classmate
'an acquaintance' is the definition.
(classmate is a kind of acquaintance)
'final check on form' is the wordplay.
'final check' becomes 'mate' (checkmate, the final check in a game of chess).
'on' is a charade indicator (letters next to each other).
'form' becomes 'class' ('class' can be a synonym of 'form'**).
'mate' after 'class' is 'CLASSMATE'.
'for' acts as a link.
(Other definitions for classmate that I've seen before include "one of the others being in form" , "someone from school?" , "Fellow pupil" , "Schoolfellow" , "Fellow student" .)