Suddenly everyone is in unison (3,2,4)

I believe the answer is:

all at once

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

'everyone is in unison' is the wordplay.
'everyone' becomes 'all' (all people).
'is in unison' becomes 'atonce' (I can't justify this - if you can you should believe this answer much more).
'all'+'atonce'='ALL AT ONCE'

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(Other definitions for all at once that I've seen before include "Without warning" , "How daffodils appeared to Wordsworth" , "in unison" , "Together" , "Simultaneously - immediately" .)

I've seen this clue in The Irish News.

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