One's successor as a swain? (8)

I believe the answer is:

follower

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'one's successor' is the definition.
'follower' can be an answer for 'successor' (thesaurus). I'm unsure of the 'one's' bit.

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

Can you help me to learn more?

(Other definitions for follower that I've seen before include "One accepts the leadership of another" , "Supporter" , "Attendant" , "Person who succeeds" , "fan" .)

I've seen this clue in the Evening Standard.

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