Hidden, clandestine (6)
I believe the answer is:
covert
I believe this is a double definition.
'hidden' is the first definition.
(similar in meaning)
'clandestine' is the second definition.
(similar in meaning)
(Other definitions for covert that I've seen before include "Hidden, secret" , "Concealed - flock of coots" , "Secret like fox's lair" , "Secret - thicket" , "Concealed, like fox's lair" .)