Finally down one’s nice spirit (3)

I believe the answer is:

nis

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'finally down one's nice spirit' is the definition.
I can't judge whether this definition defines the answer.

'finally down one's' is the wordplay.
'finally' indicates one should take the final letters.
'one's' becomes 'is' (Roman numeral).
The last letter of 'down' is 'n'.
'n'+'is'='NIS'

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I've seen this clue in The Times.

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