Accountant helping to check correct authorisation (13)
I believe the answer is:
accreditation
'authorisation' is the definition.
(accreditation is a kind of authorisation)
'accountant helping to check correct' is the wordplay.
'accountant' becomes 'acc'.
'helping' becomes 'ration' (a ration is a serving or helping).
'to check' is an insertion indicator (check can mean to restrain or block in).
'correct' becomes 'edit' (edit can mean to correct or emend).
'ration' enclosing 'edit' is 'reditation'.
'acc'+'reditation'='ACCREDITATION'
(Other definitions for accreditation that I've seen before include "Official recognition (of diplomat)" , "Granting of recognition" , "(Diplomat's) official authorisation" .)