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When there's a bit of an inundation, she's very good (3)

Ross

I believe the answer is:

nun

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'very good' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

'when there's a bit of an inundation she's' is the wordplay.
'NUN' is hidden in amongst the letters.
But, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for nun that I've seen before include "Mother Superior, perhaps" , "eg Carmelite" , "Woman respecting order" , "Black and white pigeon" , "Joshua's father" .)

I've seen this clue in the Evening Standard.
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