Mysterious coteries assembled (8)
I believe the answer is:
esoteric
'mysterious' is the definition.
(I've seen this before)
'coteries assembled' is the wordplay.
'assembled' is an anagram indicator.
'coteries' with letters rearranged gives 'ESOTERIC'.
(Other definitions for esoteric that I've seen before include "Taught to a select few" , "Intended for the few" , "Mystic" , "Secret, private" , "exclusive" .)