Politician allegedly partying in underwear? (7)

I believe the answer is:

johnson

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'politician' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see an association between them that I can't see?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for johnson that I've seen before include "American president" , "one of two presidents" , "Two US presidents; a literary Doctor" , "Lexicographical doctor" , "Samuel -, lexicographer" .)

I've seen this clue in The Independent.

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