Open secret, Charlie’s gone (5)
I believe the answer is:
overt
'open' is the definition.
(associated in meaning)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for overt that I've seen before include "Done in an obvious way" , "Clear" , "Open, not 1 across" , "Open and observable" , "Obvious, patent" .)