Crossword Genius

Stubborn person has something afoot (4)

Ross

I believe the answer is:

mule

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'stubborn person' is the definition.
(I've seen this in another clue)

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for mule that I've seen before include "Sterile offspring of donkey and horse" , "Drug smuggler" , "Donkey-horse hybrid" , "Proverbially stubborn beast" , "Drug courier; animal" .)

I've seen this clue in The Guardian.
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