As musical entertainment, one might knock it (3)
I believe the answer is:
rap
'as musical' 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?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for rap that I've seen before include "African-American music in which the lyrics are spoken" , "Smack (knuckles)" , "Knock sharply" , "Administer sharp blow, being below par" , "Sharp blow; criticism" .)