Rejecting extremists, Boris plugs new unknown faction (8)
I believe the answer is:
minority
'faction' is the definition.
The answer and definition can be both groups as well as being singular nouns.
Perhaps there's an association between them I don't understand?
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for minority that I've seen before include "Less than half" , "Relatively small group of people" , "Smaller of two groups" , "Less than 50% or 18" , "Smaller part" .)