Mystifying, perplexing (8)
I believe the answer is:
baffling
I believe this is a double definition.
'mystifying' is the first definition.
('baffle' can be a synonym of 'mystify')
'perplexing' is the second definition.
('baffle' can be a synonym of 'perplex')
(Other definitions for baffling that I've seen before include "Mystifying" , "Mysterious" , "Hard to understand" , "Perplexing, puzzling" , "Confusing, puzzling" .)