A harmonious partnership (4)
I believe the answer is:
duet
'a harmonious partnership' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps there's a link between them I don't understand?
This is all the clue.
Can you help me to learn more?
(Other definitions for duet that I've seen before include "Musical piece for two performers" , "Work for two performers" , "A song sung by two" , "Two singers performing together" , "Piece for brace" .)