Lacking ability to hold poor Enid? It's proven! (10)

I believe the answer is:

undeniable

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'proven' is the definition.
Both the definition and answer are adjectives. Maybe you can see an association between them that I can't see?

'lacking ability to hold poor enid?' is the wordplay.
'lacking ability' becomes 'unable' (I can't explain this - if you can you should give a lot more credence to this answer).
'to hold' is an insertion indicator.
'poor' indicates anagramming the letters.
'enid' is an anagram of 'deni'.
'unable' going around 'deni' is 'UNDENIABLE'.

'it's' acts as a link.

Can you help me to learn more?

(Another definition for undeniable that I've seen is " Indisputable".)

I've seen this clue in the Sydney Morning Herald.

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