If the audience can't hear it, nobody can (5)
I believe the answer is:
aside
'can' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for aside that I've seen before include "Privately, apart" , "(Pushed) out of the way" , "Separately" , "(Turn) off main path" , "Actor's remark for the audience alone" .)