Staff said leaders lost a vital piece of signed testimony (9)

I believe the answer is:

affidavit

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'testimony' is the definition.
(I know that affidavit is a type of testimony)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for affidavit that I've seen before include "Sworn written statement" , "Declaration before notary" , "legal document" , "Written evidence" , "Legal declaration" .)

I've seen this clue in the Sydney Morning Herald.

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