The phrase "operate covertly" is correct and usable in written English.
You can use it when referring to a group, organization, or individual that is secretly performing some type of activity, often for a specific purpose. For example: "The spies were instructed to operate covertly to gather intelligence from the enemy."
If they want to take risks, they have to operate covertly and break rules.
Many home-based workers operate covertly, fearing they are, or actually are, breaking some regulation or other.
In return for a North Vietnamese pledge to respect Cambodia's frontiers, he allowed Vietnamese communists to operate covertly from bases inside eastern Cambodia.
The main character, Yuri, is a 17 year-old agent in the UCCK network of technology, an organization set up by the Russian Mafia to operate covertly in the Untied States.
Doing so allows the BGF to operate covertly within the prison system.
The UN has meekly protested, but other Western organisations operate covertly anyway.
Almost all of the undercover officers identified so far – including those known to have worked under Lambert – had sexual relationships while operating covertly.
In the early days, Pirate Joe's was the unofficial name for the enterprise, which Hallatt operated covertly from what looked like a rundown bakery.
In retaliation, the Israeli Air Force, which has been operating covertly in Syria since 2012, sent F-16 fighter jets to attack an Iranian command center in central Syria.
Unlike the drones the C.I.A. operated covertly across the border in Pakistan, this aircraft was one of nearly a dozen military drones patrolling vast stretches of hostile Afghan territory on any given day.
In late August, Nato estimated that more than 1,000 Russian soldiers were operating covertly inside Ukraine, as part of a major separatist effort to roll back advances by Kiev.
