Mysterious coteries formed (8)
I believe the answer is:
esoteric
'mysterious' is the definition.
(I've seen this before)
'coteries formed' is the wordplay.
'formed' is an anagram indicator.
'coteries' is an anagram of 'ESOTERIC'.
(Other definitions for esoteric that I've seen before include "I set core up to be mysterious" , "Other-worldly" , "Hard to understand" , "Intelligible only to the initiated" , "Intended for the few" .)