The phrase "highly covert" is correct and can be used in written English.
It is typically used to describe something that is hidden or disguised. For example, "The spies were able to pass the documents without detection due to their highly covert methods."
What began as highly specialized, covert tool to locate and kill one individual has developed into today's default counterterrorism tactic.
He's doing it because the men under his command, a highly secret covert operations unit, didn't receive public honours when they died in combat, as if that wouldn't defeat the whole point of a highly secret covert operations unit – but the government agents trying to stop the man can't decide if he's a "legend", a "hero", or only a "man of honour".
Mixed martial arts fighter and American Gladiators participant Gina Carano is featured in her first starring role as Mallory Kane, a highly-skilled covert ops specialist who discovers that she is being double-crossed by those she has trusted with her life.
Mrs. Clinton, as she did earlier this week, played down concerns about North Korea's having a covert, highly enriched uranium program and said too much focus on it could distract from the need to confront North Korea over its publicly declared nuclear activities.
Traditionally, we've perceived 3D printing as an almost gimmicky, novel practice that thrives in highly funded and covert research labs.
Although no one in Israel is leaking any details, it's understood that intelligence agencies -- led by the foreign operations agency, the Mossad -- are highly active in covert operations against Iran's nuclear work.
By the fall of '69 the President's assault on the press & the TV networks had grown into an elaborate, highly organized overt & covert program of intimidation.
