Assembly, convention (6)
I believe the answer is:
caucus
I believe this clue is a double definition.
'assembly' is the first definition.
(I've seen this before)
'convention' is the second definition.
(thesaurus)
(Other definitions for caucus that I've seen before include "Small, dominant political group" , "Small group in branch of political party making plans, work may be used scornfully" , "Small group of like-minded politicians" , "Policy meeting" , "Meeting of party leaders" .)