To claim post, erstwhile insiders committed fraud? (9)

I believe the answer is:

imposters

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'insiders committed fraud?' is the definition.
I can't judge whether this definition defines the answer.

'to claim post erstwhile' is the wordplay.
'IMPOSTERS' is hidden in amongst the letters.
But, I am uncertain how this is indicated.

Can you help me to learn more?

(Other definitions for imposters that I've seen before include "They assume identities in order to deceive" , "Fakes" .)

I've seen this clue in The Irish Times.

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