Abstruse, arcane (8)
I believe the answer is:
esoteric
I believe this is a double definition.
'abstruse' is the first definition.
(I've seen this before)
'arcane' is the second definition.
(similar in meaning)
(Other definitions for esoteric that I've seen before include "Secret, mysterious" , "known to a few" , "exclusive" , "Occult" , "Intended for the few" .)