Assent to treaty accompanied by noises off (9)
I believe the answer is:
accession
'assent to treaty' is the definition.
(accession is agreeing to a treaty)
'accompanied by noises off' is the wordplay.
'accompanied' becomes 'acc' (abbreviation).
'by' says to put letters next to each other.
'off' is an anagram indicator.
'noises' with letters rearranged gives 'ession'.
'acc'+'ession'='ACCESSION'
(Other definitions for accession that I've seen before include "New item added to a library or museum" , "Act of taking up office" .)