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She's vowed to be good (3)

Ross

I believe the answer is:

nun

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'good' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for nun that I've seen before include "Mother Superior, perhaps" , "Devout woman" , "One who's taken vows" , "Convent dweller" , "Woman respecting order" .)

I've seen this clue in the Evening Standard.
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