Logo for a conservative authority? (9)

I believe the answer is:

signatory

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

'authority?' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe you can see an association between them that I can't see?

'logo for a conservative' is the wordplay.
'logo for' becomes 'sign' (I can't justify this - if you can you should believe this answer much more).
'conservative' becomes 'tory' (informal term for a Conservative Party member).
'sign'+'a'+'tory'='SIGNATORY'

Can you help me to learn more?

(Another definition for signatory that I've seen is " one in agreement".)

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

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