Religious debt put in postscript (5)
I believe the answer is:
pious
'religious' is the definition.
('pious' can be similar in meaning to 'religious')
'debt put in postscript' is the wordplay.
'debt' becomes 'iou'.
'put in' means one lot of letters goes inside another.
'postscript' becomes 'ps' ('PS' can be a synonym of 'postscript').
'iou' placed into 'ps' is 'PIOUS'.
(Other definitions for pious that I've seen before include "God-fearing" , "Sanctimonious" , "Saintly" , "Godly" , "Holier-than-thou" .)