Influential group causes custom to be observed only on the left (6)
I believe the answer is:
caucus
'left' is the definition.
The definition and answer can be both groups as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for caucus that I've seen before include "Small group in political party making plans, policies etc" , "Closed political meeting" , "Small group of like-minded politicians" , "Small group in branch of political party making plans, work may be used scornfully" , "Political gathering to select a candidate, US-style" .)