Britain’s right to quit EU — net tax burden after negotiation is not diminished (12)

I believe the answer is:

unextenuated

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'britain's right to quit eu net tax burden after negotiation' is the definition.
I can't tell whether this defines the answer.

'not diminished' is the wordplay.
'not' becomes 'un' (I've seen this in other clues).
'diminished' becomes 'extenuated' (I've seen this in another clue).
'un'+'extenuated'='UNEXTENUATED'

'is' acts as a link.

Can you help me to learn more?

I've seen this clue in The Hindu.

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