Sinful rascal promises to pay (7)
I believe the answer is:
impious
'sinful' is the definition.
(thesaurus)
'rascal promises to pay' is the wordplay.
'rascal' becomes 'imp' ('imp' can be a synonym of 'rascal').
'promises to pay' becomes 'ious' (more than one iou).
'imp'+'ious'='IMPIOUS'
(Other definitions for impious that I've seen before include "Ungodly" , "Lacking reverence for God" , "Irreverent; wicked" , "Lacking veneration or respect" , "Showing disrespect for a god" .)