something hidden or kept secret
clandestine - secret
The word ‘clandestine’ is used to express something done in secret, especially if it is not officially permitted or liable to get you into trouble. For that reason, the term is often associated with things like espionage, plots, or illicit love affairs.
That spy organization undertook a number of clandestine operations during the war.
Richard was fired because it was discovered that he had been having a clandestine affair with his secretary for several years.
The plotters used the basement of the building to conduct their clandestine meetings.