Cryptic from some clever expert, being extremely selective (6)
I believe the answer is:
secret
'cryptic' is the definition.
(synonyms)
I don't understand how the rest of the clue works.
(Other definitions for secret that I've seen before include "Private, or something private" , "Not public" , "Security classification level" , "It's not to be told" , "Kept under wraps" .)