Clandestine operation
A clandestine operation was an espionage or military operation planned and performed in secret.

In 2399, Jean-Luc Picard had information about clandestine Romulan anti-synth ops on Earth. General Oh, a Tal Shiar  in Starfleet, told Admiral Kirsten Clancy that if the Romulans were running clandestine operations on Earth, that she, then Clancy, and then all of Starfleet would know because it would be an overt and intolerable act of war. ()

See also 
 Black ops

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