Covert, secret (11)
I believe the answer is:
clandestine
I believe this is a double definition.
'covert' is the first definition.
(similar in meaning)
'secret' is the second definition.
(I know that secret can be written as clandestine)
(Other definitions for clandestine that I've seen before include "backroom" , "Done secretly" , "Undercover" , "Private" , "Secret" .)