Second religious summary, a lengthy read (6)
I believe the answer is:
screed
'a lengthy read' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'second religious summary' is the wordplay.
'second religious' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'summary' becomes 'creed' (I have seen 'Summary of articles' mean 'creed' so perhaps 'summary' could also mean 'creed').
's'+'creed'='SCREED'
Can you help me to learn more?
(Other definitions for screed that I've seen before include "Lengthy statement" , "Tediously long speech" , "A long written effusion" , "Tiresomely long document about creeds" , "Tiresomely long letter about creeds" .)