Wicked rogue promises to pay (7)
I believe the answer is:
impious
'wicked' is the definition.
(associated in meaning)
'rogue promises to pay' is the wordplay.
'rogue' becomes 'imp' ().
'promises to pay' becomes 'IOU's' (promises to pay a debt).
'imp'+'ious'='IMPIOUS'
(Other definitions for impious that I've seen before include "Irreverent; wicked" , "Lacking reverence for God" , "Lacking veneration or respect" , "Showing disrespect for a god" , "9 [DEMON]" .)