Ravens rummaging in nun’s desk (10)

I believe the answer is:

unkindness

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

'ravens rummaging' is the definition.
I can't tell whether this definition defines the answer.

'in nun's desk' is the wordplay.
'in nun's desk' can be anagrammed to 'UNKINDNESS'.
Though, I am not clear how the anagram is indicated.

Can you help me to learn more?

(Another definition for unkindness that I've seen is " spite".)

I've seen this clue in the Sydney Morning Herald.

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