Intelligible only to the initiated (8)
I believe the answer is:
esoteric
'intelligible only to the initiated' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for esoteric that I've seen before include "Abstruse; secret" , "Secret, private" , "Privileged or confidential among coteries" , "Mysterious to all but a chosen few" , "Aimed at a few" .)