Former pupil against debts being beyond dispute (7)
I believe the answer is:
obvious
'being beyond dispute' is the definition.
(beyond dispute means obvious or undeniable)
'former pupil against debts' is the wordplay.
'former pupil' becomes 'OB' (abbreviation for 'Old Boy').
'against' becomes 'v' (abbreviation for 'versus').
'debts' becomes 'IOUs' (promises to pay debts).
'ob'+'v'+'ious'='OBVIOUS'
(Other definitions for obvious that I've seen before include "Plain to see" , "Clear as day" , "Plain to be seen" , "Quite clear and manifest" , "Patent" .)