Unexpected gains with politicians — those endorsing proposal? (11)

I believe the answer is:

signatories

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

'those endorsing proposal?' is the definition.
I can't judge whether this defines the answer.

'unexpected gains with politicians' is the wordplay.
'unexpected' indicates an anagram.
'with' is a charade indicator (letters next to each other).
'politicians' becomes 'tories' (informal term for the Conservative Party).
'gains' anagrammed gives 'signa'.
'signa'+'tories'='SIGNATORIES'

Can you help me to learn more?

(Another definition for signatories that I've seen is " Witnesses".)

I've seen this clue in The Times.

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