A, B, C, D, F or G? (4)

I believe the answer is:

note

I'm a little stuck... Click here to teach me more about this clue!

'a b c' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe they are linked in a way I don't understand?

I don't understand how the rest of the clue works.

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" .)

I've seen this clue in The Sun, The Independent and The Times.

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