He has the worst part that's just not his! (4)

I believe the answer is:

hers

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'not his' is the definition.
(I've seen this in another clue)

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

(Other definitions for hers that I've seen before include "The woman's" , "Belonging to the female" .)

I've seen this clue in the Evening Standard.

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