A small letter A? (4)

I believe the answer is:

note

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'a small letter' is the definition.
(I've seen this in another clue)

I can't explain the rest of the clue.

(Other definitions for note that I've seen before include "A flat, for example" , "Do, re or mi?" , "Quaver, say" , "Observe the musical pitch." , "A, B or C, for example - short message" .)

I've seen this clue in The Times and The Sunday Times.

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