Devout priest promises to pay (5)
I believe the answer is:
pious
'devout' is the definition.
('pious' can be a synonym of 'devout')
'priest promises to pay' is the wordplay.
'priest' becomes 'p' (ecclesiastical abbreviation).
'promises to pay' becomes 'ious' (more than one iou).
'p'+'ious'='PIOUS'
(Other definitions for pious that I've seen before include "Dutiful" , "Holier-than-thou" , "Devout" , "Self-righteous" , "Godly" .)