Religious debt put in the postscript (5)
I believe the answer is:
pious
'religious' is the definition.
(associated in meaning)
'debt put in the postscript' is the wordplay.
'debt' becomes 'iou' (I owe you, a promise to pay a debt).
'put in' means one lot of letters goes inside another.
'the postscript' becomes 'ps' (abbreviation).
'iou' going within 'ps' is 'PIOUS'.
(Other definitions for pious that I've seen before include "Sanctimonious" , "Devoutly religious" , "Devout" , "Godly" , "Self-righteous" .)