Bob rules Aunt Sam out, very obstinate (8,2,1,4)

I believe the answer is:

stubborn as a mule

'very obstinate' is the definition.
(I've seen this in another clue)

'bob rules aunt sam out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'bob'+'rules'+'aunt'+'sam'='bobrulesauntsam'
'bobrulesauntsam' anagrammed gives 'STUBBORN AS A MULE'.

I've seen this clue in The Daily Express.

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