Stubborn stain to be treated (9)

I believe the answer is:

obstinate

'stubborn' is the definition.
(I know that stubborn can be written as obstinate)

'stain to be treated' is the wordplay.
'treated' indicates an anagram.
'stain'+'to'+'be'='staintobe'
'staintobe' is an anagram of 'OBSTINATE'.

(Other definitions for obstinate that I've seen before include "Blindly stubborn" , "Female bird" , "Pig-headed" , "Stubborn, obdurate" , "Stubborn or unyielding" .)

I've seen this clue in The Sun, The Telegraph, The Independent and The Guardian.

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