The phrase "clandestine activities" means secret or hidden actions that are done in a discreet or undercover manner, not meant to be known or discovered by others. Full definition
«Without the economic assistance of the U.S. we could never have exercised or organized or carried out clandestine activities of this kind. (religion-online.org)
When the author began covering bitcoin back in 2013, cryptocurrency was synonymous with dark web criminal overlords, hackers and other clandestine activities. (hacked.com)
Arendt's attitude toward the Congress for Cultural Freedom, an informal gathering of anti- and ex-communists, indicates how severely critical she was of such CIA - sponsored clandestine activity in the early 1950s. (firstthings.com)