Not widely understood (6)
I believe the answer is:
arcane
'not widely understood' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for arcane that I've seen before include "Mysteriouss or secret" , "Ace ran, being mysterious" , "Recondite" , "Abstruse, mysterious" , "Requiring special knowledge" .)