Successful man promises to pay (10)
I believe the answer is:
victorious
'successful' is the definition.
('victorious' can be similar in meaning to 'successful')
'man promises to pay' is the wordplay.
'man' becomes 'victor' (**).
'promises to pay' becomes 'ious' (more than one iou).
'victor'+'ious'='VICTORIOUS'
(Other definitions for victorious that I've seen before include "winning" , "Triumphant" .)