In on (with "to”) (5)
I believe the answer is:
privy
'in on with to' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for privy that I've seen before include "somewhere to go" , "Garden shed" , "Admitted (to secrets)" , "Sharing in secret knowledge - toilet" , "Participating in the knowledge of something secret" .)