Starting from all under-plumage to tumble (4,4)

I believe the answer is:

fall down

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'to tumble' is the definition.
(I've seen this before)

'starting from all under-plumage' is the wordplay.
'starting' says to take the initial letters.
'under plumage' becomes 'down' (I can't explain this - if you can you should believe this answer much more).
The first letter of 'from' is 'f'.
'f'+'all'+'down'='FALL DOWN'

Can you help me to learn more?

(Other definitions for fall down that I've seen before include "Collapse" , "Tumble" .)

I've seen this clue in the Sydney Morning Herald.

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