Suddenly everyone is in unison (3,2,4)
I believe the answer is:
all at once
'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'
Can you help me to learn more?
(Other definitions for all at once that I've seen before include "Without warning" , "How daffodils appeared to Wordsworth" , "in unison" , "Together" , "Simultaneously - immediately" .)