Prime Minister's willy (7)

I believe the answer is:

johnson

I believe this is a double definition.

'prime minister's' is the first definition.
(Boris Johnson)

'willy' is the second definition.
(both are slang terms for penis)

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

I've seen this clue in The Independent.

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