Clandestine (6)
There are a few possibilities for this:
hugger-mugger
'clandestine' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for secret that I've seen before include "Kept hidden from others" , "Unidentified" , "Confidence" , "Concealed" , "Esoteric" .)
'clandestine' is the definition.
(similar in meaning)
This is the entire clue.
(Other definitions for covert that I've seen before include "18 [HIDDEN]" , "Concealed - vector (anag)" , "Concealed - flock of coots" , "Hidden, secret" , "Thicket; not displayed" .)
'clandestine' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for hugger-mugger that I've seen before include "(In) confusion" , "In secret - disorderly ..." , "In utter disorder - in secret" .)