WordNet
n. an operation involving secret work within a community or institution
Wikipedia
To go "undercover" is to avoid detection by the entity one is observing, and especially to disguise one's own identity or use an assumed identity for the purposes of gaining the trust of an individual or organization to learn or confirm confidential information or to gain the trust of targeted individuals in order to gather information or evidence. Traditionally, it is a technique employed by law enforcement agencies or private investigators, and a person who works in such a role is commonly referred to as an undercover agent.
Usage examples of "undercover operation".
They also knew the Americans were running a sensitive undercover operation, and must have assumed the Americans would want to handle the situation themselves.
Concerned that in my disaffected state I might be vulnerable to recruitment by South African counter-intelligence, they pulled my friend Milton out of the country and cancelled the whole undercover operation.
We had gotten together today so that I could hand over documents- the final, signed copy of Robbie's plea agreement and his acknowledgment that he'd reviewed the lengthy written protocol for the undercover operation UCORC had generated- both of which were hidden in the folds of the morning's newspaper.
He should have had enough foresight instead of hindsight not to have let Martha and Aideen undertake what amounted to an undercover operation without a shadow or two, someone to watch their backs.
Four times a day, when tracking an undercover operation, one or more aircraft belonging to AMEXICO, and of a type unknown to Steve, flew back and forth on preset flight-paths, with electronic ears tuned to the ground below.
You've come very close to destroying a five-year undercover operation we've had going against this bunch.
Someone who knew about the undercover operation passed the information to the other side.