Like a mule turning 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.

'turning into beast' is the wordplay.
'turning' is an anagram indicator.
'into'+'beast'='intobeast'
'intobeast' with letters rearranged gives 'OBSTINATE'.

Can you help me to learn more?

(Other definitions for obstinate that I've seen before include "headstrong" , "such as a mule" , "Pig-headed" , "Stubborn or unyielding" , "Like mules" .)

I've seen this clue in The Telegraph.

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