Debatable topic broached after beastly noise (4)
I believe the answer is:
moot
'debatable' is the definition.
('moot' can be a synonym of 'debatable')
'topic broached after beastly noise' is the wordplay.
'broached' suggests taking the first letters.
'after' is a charade indicator (letters next to each other).
'beastly noise' becomes 'moo'.
The first letter of 'topic' is 't'.
't' put after 'moo' is 'MOOT'.
(Other definitions for moot that I've seen before include "Undecided as a debatable point may be" , "raise for discussion" , "Open to question" , "Questionable, debatable" , "Propose for discussion" .)