done or kept secret in inglese:

clandestine marriage

nothing is quite as puzzling as our predilection for clandestine copulation.

Announcing to her boyfriend that she was going to the library, Maria actually went to meet George for a clandestine liaison.

If we are rejecting illegal, clandestine immigration, it is because of all the trafficking...

the clandestine group / clandestine love affair

They were having a clandestine affair for 4 years.