Like a mule, transforming into beast (9)

I believe the answer is:

obstinate

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'like a mule' is the definition.
I can't tell whether this definition defines the answer.

'transforming into beast' is the wordplay.
'transforming' indicates anagramming the letters (I've seen this in other clues).
'into'+'beast'='intobeast'
'intobeast' is an anagram of 'OBSTINATE'.

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(Other definitions for obstinate that I've seen before include "Firm" , "Pig-headed" , "such as a mule" , "Stubborn, obdurate" , "Stubbornly persistent" .)

I've seen this clue in The Daily Express.

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