Possible swinger with a hangup! (3)

I believe the answer is:

bob

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'possible' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for bob that I've seen before include "line of verse" , "Hairstyle cut shortly and evenly all round" , "Set of changes" , "-- Dylan, singer" , "Old coin for Robert" .)

I've seen this clue in the Evening Standard.

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