Biblical teaching by old Luther, for one (8)
I believe the answer is:
reformer
'biblical teaching' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'by old luther for one' is the wordplay.
'by' is a charade indicator (letters next to each other).
'old' becomes 'former' (eg a former president, an old president).
'luther for one' becomes 're' (I can't justify this - if you can you should believe this answer much more).
'former' put after 're' is 'REFORMER'.
Can you help me to learn more?
(Other definitions for reformer that I've seen before include "Luther, for one" , "One making improving changes" , "campaigner for change" , "Person who advocates change" , "One who changes organisation in order to improve" .)