Hidden, covert (6)
I believe the answer is:
secret
I believe this is a double definition.
'hidden' is the first definition.
(I know that hidden can be written as secret)
'covert' is the second definition.
(similar in meaning)
(Other definitions for secret that I've seen before include "Private, or something private" , "Esoteric" , "One not to be divulged" , "Mystery" , "Unknown or unobserved by others" .)