Element of expanded account? (8)

I believe the answer is:

actinium

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

'element' is the definition.
(actinium is a kind of element)

'expanded account?' is the wordplay.
I cannot really see how this works, but
'account?' could be 'act' (account is a kind of act) and 'act' is present in the answer.
This may be the basis of clue (or it may be nonsense).

'of' is the link.

Can you help me to learn more?

(Another definition for actinium that I've seen is " Radioactive metallic element".)

I've seen this clue in The Telegraph.

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