A, B, C, D, F or G? (4)
I believe the answer is:
note
'a b c' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for note that I've seen before include "Eg, minim" , "Cash" , "Eminence" , "Comment" , "Pay close attention to" .)