Agonising end of bird is prohibited (9)

I believe the answer is:

forbidden

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'is prohibited' is the definition.
(synonyms)

'agonising end of bird' is the wordplay.
'end of bird' anagrams to 'FORBIDDEN'.
However, I am not clear how this is indicated.

Can you help me to learn more?

(Other definitions for forbidden that I've seen before include "Disallowed, banned" , "Illicit pleasure" , "not 35 [PERMISSIBLE]" , "Verboten, prohibited" , "This fruit is always more enticing" .)

I've seen this clue in The Globe and Mail.

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