Devout Penny promises to pay (5)
I believe the answer is:
pious
'devout' is the definition.
('pious' can be a synonym of 'devout')
'penny promises to pay' is the wordplay.
'penny' becomes 'p'.
'promises to pay' becomes 'ious' (more than one iou).
'p'+'ious'='PIOUS'
(Other definitions for pious that I've seen before include "Godly" , "Dutiful" , "Saintly" , "Devout" , "Good" .)