Occult (8)
I believe the answer is:
esoteric
'occult' is the definition.
(similar in meaning)
This is all the clue.
(Other definitions for esoteric that I've seen before include "Secret, private" , "known to a few" , "I set core up to be mysterious" , "Understandable by only an enlightened inner circle" , "Secret kind of coteries" .)