One involved in plot? (6)
I believe the answer is:
reader
'one involved in plot?' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe you can see a link between them that I can't see?
This is all the clue.
Can you help me to learn more?
(Other definitions for reader that I've seen before include "Person studying a book" , "He's immersed in a book or newspaper" , "Publisher's selector" , "University lecturer - text book" , "Peruser of text" .)