Resolution’s extremely decisive conclusion (13)

I believe the answer is:

determination

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'resolution's' is the definition.
(determination is a kind of resolution)

'extremely decisive conclusion' is the wordplay.
'extremely decisive' becomes 'de' (I can't explain this - if you can you should give a lot more credence to this answer).
'conclusion' becomes 'termination' (synonyms).
'de'+'termination'='DETERMINATION'

Can you help me to learn more?

(Other definitions for determination that I've seen before include "Resolution" , "Decision; grit" , "legal decision" , "Perseverance" , "24 [DRIVE]" .)

I've seen this clue in The Times.

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