I'm about to shake after passing brief note

I believe the answer is:

demisemiquaver

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

'brief note' is the definition.
'demisemiquaver' can be an answer for 'note' (demisemiquaver is a kind of note). I am unsure of the 'brief' bit.

'i'm about to shake after passing' is the wordplay.
'about' says the letters should be written in reverse.
'to shake' becomes 'quaver' (I've seen this in another clue).
'after' means one lot of letters go next to another.
'passing' becomes 'demise' (I've seen this before).
'im' written backwards gives 'mi'.
'mi'+'quaver'='miquaver'
'miquaver' put after 'demise' is 'DEMISEMIQUAVER'.

Can you help me to learn more?

(Another definition for demisemiquaver that I've seen is " making very little sound".)

I've seen this clue in The Daily Mail.

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