Supporting business, one is protected by Conservatives (12)

I believe the answer is:

confirmatory

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'supporting' is the definition.
(thesaurus)

'business one is protected by conservatives' is the wordplay.
I cannot really see how this works, but
'business' could be 'firm' (firm is a kind of business) and 'firm' is found in the answer.
'one' could be 'o' (I've seen this in other clues) and 'o' is present in the answer.
'by' could be 'at' and 'at' is found in the answer.
'conservatives' could be 'c' (abbreviation in UK politics) and 'c' is found within the answer.
This explanation may well be incorrect...

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 Guardian.

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