Do, re, me, fa, so or la? (4)

I believe the answer is:

note

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'do' 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?

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

Can you help me to learn more?

(Other definitions for note that I've seen before include "mention" , "Brief letter - distinction" , "Heed; short letter" , "Observe; paper money" , "Make a mental or written reminder" .)

I've seen this clue in The Times.

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