Secrets, mysteries (6)
I believe the answer is:
arcana
I believe this clue is a double definition.
'secrets' is the first definition.
('arcanum' can be a synonym of 'secret')
'mysteries' is the second definition.
(I've seen this before)
(Other definitions for arcana that I've seen before include "Secrets, mysteries" , "Secret things" , "Tarot cards" .)