Secret; obscure (6)
I believe the answer is:
arcane
This clue is a double definition.
'secret' is the first definition.
(I've seen this before)
'obscure' is the second definition.
(I've seen this in another clue)
(Other definitions for arcane that I've seen before include "Mysterious, understood by few" , "Ace ran, being mysterious" , "Abstruse, mysterious" , "Esoteric, mysterious" , "Requiring secret or mysterious knowledge" .)