Charlie's open secret (6)
I believe the answer is:
covert
'secret' is the definition.
('covert' can be similar in meaning to 'secret')
'charlie's open' is the wordplay.
'charlie' becomes 'c' (phonetic alphabet: alpha, bravo, charlie etc.).
'open' becomes 'overt' ('overt' is associated in meaning with 'open').
'c'+'overt'='COVERT'
(Other definitions for covert that I've seen before include "Hidden, secret" , "Concealed, like fox's lair" , "Unacknowledged; thicket" , "May hold the game" , "Concealed - vector (anag)" .)