Everyone in unison suddenly (3,2,4)
I believe the answer is:
all at once
'suddenly' is the definition.
(I've seen this before)
'everyone in unison' is the wordplay.
'everyone' becomes 'all' (all people).
'in unison' becomes 'atonce' (I am not sure about this - if you are sure 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 "Simultaneously - immediately" , "Suddenly" , "Without warning" , "Together" , "How daffodils appeared to Wordsworth" .)