Church books go on about Old Nick (5)
I believe the answer is:
notch
'nick' is the definition.
(both can mean a small cut or impression)
'church books go on about old' is the wordplay.
I cannot quite understand how this works, but
'church' could be 'ch' (common abbreviation) and 'ch' is found in the answer.
'books' could be 'ot' (Old Testament) and 'ot' is found within the answer.
A single letter 'n' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for notch that I've seen before include "Tallying incision" , "V-shaped nick" , "Nick - score" , "Nick - incision" , "Small V-shaped cut or indentation" .)