The start of a banned clique (6)
I believe the answer is:
outset
'the start' is the definition.
(synonyms)
'banned clique' is the wordplay.
'banned' becomes 'out'.
'clique' becomes 'set' (a clique is a social set).
'out'+'set'='OUTSET'
'of a' acts as a link.
(Other definitions for outset that I've seen before include "origin" , "Opening stage" , "Inception" , "Set out in the beginning" , "Starting point" .)