Addendum to reform church text (7)
I believe the answer is:
psalter
'church text' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'addendum to reform' is the wordplay.
'addendum' becomes 'ps' (I can't explain this - if you can you should give a lot more credence to this answer).
'to reform' becomes 'alter' (I've seen this before).
'ps'+'alter'='PSALTER'
Can you help me to learn more?
(Other definitions for psalter that I've seen before include "Church chanting book" , "sacred work" , "Books of Psalms" , "Book used in church" , "Book for church use" .)