Obscure, difficult to understand (8)
I believe the answer is:
abstruse
I believe this clue is a double definition.
'obscure' is the first definition.
(synonyms)
'difficult to understand' is the second definition.
(I've seen this before)
(Other definitions for abstruse that I've seen before include "Hard to understand, recondite" , "Hard to understand, perhaps profound" , "Incomprehensible to those with normal understanding" , "Hard to understand, complicated" , "Difficult to comprehend" .)