Suddenly acting together, as a group (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 it can be defined by this definition.

'acting together as a group' is the wordplay.
I cannot really see how this works, but
'acting' could be 'on' (I've seen this in other clues) and 'on' is found in the answer.
'a' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Irish Times.

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