Note I tremble after death march's beginning (14)
I believe the answer is:
demisemiquaver
'march's beginning' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'note i tremble after death' is the wordplay.
'note i' becomes 'mi' (I can't justify this - if you can you should believe this answer much more).
'tremble' becomes 'quaver' (I've seen this in another clue).
'after' is a charade indicator (letters next to each other).
'death' becomes 'demise' (synonyms).
'mi'+'quaver'='miquaver'
'miquaver' after 'demise' is 'DEMISEMIQUAVER'.
Can you help me to learn more?
(Another definition for demisemiquaver that I've seen is " making very little sound".)