Unpleasant behaviour in nests is an absurdity (9)

I believe the answer is:

nastiness

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

'unpleasant behaviour' is the definition.
(I've seen this before)

'nests is an absurdity' is the wordplay.
'nests is an' anagrams to 'NASTINESS'.
But, I am not sure how this is indicated.

'in' is the link.

Can you help me to learn more?

(Other definitions for nastiness that I've seen before include "hostility" , "Repugnant behaviour" , "repugnance" , "Unpleasant behaviour" , "ill humour" .)

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

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