Joining a couple of hundred, accompanied by rebellious noises (9)
I believe the answer is:
accession
'joining' is the definition.
(thesaurus)
'a couple of hundred accompanied by rebellious noises' is the wordplay.
'couple of hundred' becomes 'CC' (Roman numerals for 200).
'accompanied by' is a charade indicator (letters next to each other).
'rebellious' is an anagram indicator.
'noises' anagrammed gives 'ession'.
'a'+'cc'+'ession'='ACCESSION'
(Other definitions for accession that I've seen before include "Act of taking up office" , "New item added to a library or museum" .)