Cash in on his missing list of quality books (5)
I believe the answer is:
canon
'list of quality books' 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?
'cash in on his missing' is the wordplay.
'missing' is a deletion indicator.
'cash'+'in'+'on'='cashinon'
'cashinon' with 'his' removed is 'CANON'.
Can you help me to learn more?
(Other definitions for canon that I've seen before include "General rule; clergyman" , "collected works" , "General principle; musical round" , "Standard" , "Part of chapter" .)