Extra piece the author put in letter, section of second letter or third (3)
I believe the answer is:
psi
'third' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'extra piece the author put in letter section of second letter' is the wordplay.
I cannot really understand how this works, but
'piece' could be 'p' (abbreviation for Pawn) and 'p' is found within the answer.
'letter' could be 's' and 's' is found within the answer.
A single letter 'i' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'or' is the link.
(Other definitions for psi that I've seen before include "one employed by Socrates?" , "Greek letter; pounds per square inch (abbrev.)" , "letter from abroad" , "foreign letter" , "Greek letter; pressure unit" .)