Opposing company, a politician offering convincing evidence? (12)

I believe the answer is:

confirmatory

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'opposing company a politician offering convincing evidence?' is the definition.
I can't judge whether this definition defines the answer.

'opposing company a politician' is the wordplay.
'opposing' becomes 'con' (I've seen this in another clue).
'company' becomes 'firm' (firm can mean a company or business).
'politician' becomes 'tory' (term for a Conservative Party politician).
'con'+'firm'+'a'+'tory'='CONFIRMATORY'

Can you help me to learn more?

(Another definition for confirmatory that I've seen is " Providing evidence for".)

I've seen this clue in The Times.

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