A little letter which isn't the fifth? (4)

I believe the answer is:

note

This clue could be a double definition.

'a little letter which isn't' is the first definition.
'note' can be an answer for 'a little letter' (short letter or written communication). I'm unsure of the rest of the definition.

'the fifth?' is the second definition.
(thesaurus)

(Other definitions for note that I've seen before include "Memo; tone" , "Symbol in a score" , "Observe; paper money" , "Observe in Eton" , "Memo; piano key" .)

I've seen this clue in The Times.

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