Mysterious coteries abroad (8)
I believe the answer is:
esoteric
'mysterious' is the definition.
('esoteric' can be similar in meaning to 'mysterious')
'coteries abroad' is the wordplay.
'abroad' indicates an anagram.
'coteries' anagrammed gives 'ESOTERIC'.
(Other definitions for esoteric that I've seen before include "exclusive" , "Secret, private" , "Privileged or confidential among coteries" , "just seen by specialist" , "needing privy" .)