Doctor: ' … er, you're still lacking determination' (12)

I believe the answer is:

irresolutely

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'doctor er you're still lacking determination' is the definition.
I can't judge whether this definition defines the answer.

'doctor er you're still' is the wordplay.
'doctor ' indicates an anagram.
'er'+'youre'+'still'='eryourestill'
'eryourestill' is an anagram of 'irresolutely'.

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I've seen this clue in The Guardian.

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