Secret coteries formed (8)
I believe the answer is:
esoteric
'secret' is the definition.
('esoteric' can be similar in meaning to 'secret')
'coteries formed' is the wordplay.
'formed' indicates an anagram.
'coteries' is an anagram of 'ESOTERIC'.
(Other definitions for esoteric that I've seen before include "unintelligible to outsiders" , "Aimed at a few" , "exclusive" , "Cryptic" , "known to a few" .)