This is the 10184th most frequent Spanish word.
clandestino
Secret, illegal, or hidden.
Here, 'clandestina' is used as an adjective describing the manner in which 'they traveled', indicating secrecy or hiddenness.
Viajaban de manera clandestina para evitar ser descubiertos.
They traveled clandestinely to avoid being discovered.
In this context, 'clandestinas' is again an adjective that describes the meetings, denoting they were secretive and undisclosed.
El grupo tenía reuniones clandestinas en el sótano.
The group held clandestine meetings in the basement.
Here, 'clandestino' serves as an adjective to illustrate that the laboratory's activities were concealed and unauthorized.