Admitted a vendor initially unpaid (6)
I believe the answer is:
avowed
'admitted' is the definition.
(avowing is a kind of admiting)
'a vendor initially unpaid' is the wordplay.
'initially' suggests taking the first letters.
'unpaid' becomes 'owed' ('owed' can be similar in meaning to 'unpaid').
The initial letter of 'vendor' is 'v'.
'a'+'v'+'owed'='AVOWED'
(Other definitions for avowed that I've seen before include "Professed" , "Openly asserted" , "Acknowledged" , "Confessed openly" , "Declared" .)