Knowing, as a secret (2,2)
I believe the answer is:
in on
'knowing as a secret' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for in on that I've seen before include "Wise to" , "Aware of secret" , "Sharing with" , "Aware of - being part of" , "Privy to" .)