Esoteric, recondite (6)
I believe the answer is:
arcane
I believe this clue is a double definition.
'esoteric' is the first definition.
(similar in meaning)
'recondite' is the second definition.
(I've seen this before)
(Other definitions for arcane that I've seen before include "Requiring secret or mysterious knowledge" , "Mysterious, secret or obscure" , "Ace ran, being mysterious" , "Esoteric, mysterious" , "Mysterious like a crane" .)