Crossword Genius

They are utterly conclusive (4,5)

Ross

I believe the answer is:

last words

I'm a little stuck... Click here to teach me more about this clue!

'they are utterly' is the definition.
I know nothing about this answer so I cannot judge whether this works.

'conclusive' is the wordplay.
I cannot quite understand how this works, but
'conclusive' could be 'last' (similar in meaning) and 'last' is found in the answer.
The remaining letters 'words' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Globe and Mail.
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