Suddenly, everything simultaneously (9)

I believe the answer is:

allatonce

I'm a little stuck... Click here to teach me more about this clue!

'suddenly' is the definition.
I know nothing about this answer so I can't tell whether this works.

'everything simultaneously' is the wordplay.
'everything' becomes 'all' (all things).
'simultaneously' becomes 'at once' (I've seen this before).
'all'+'atonce'='ALLATONCE'

Can you help me to learn more?

I've seen this clue in The Daily Express.

Want a hint initially instead of a full solution? Install my app