Suddenly everybody was together (3,2,4)
I believe the answer is:
all at once
'suddenly' is the definition.
(I've seen this before)
'everybody was together' is the wordplay.
'everybody' becomes 'all' (all people).
'was together' becomes 'atonce' (I can't explain this - if you can you should give a lot more credence to this answer).
'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" , "Together" , "in unison" , "Simultaneously - immediately" , "How daffodils appeared to Wordsworth" .)