New recruit about to go up inside arboreal breeding place (7)

I believe the answer is:

rookery

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'breeding place' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for rookery that I've seen before include "breeding ground" , "Nesting-place" , "Lofty breeding-place" , "Colony of penguins or seals" , "Community of birds" .)

I've seen this clue in The Guardian.

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