The phrase "a sort of underground" is correct and usable in written English.
It can be used to describe something that is not mainstream or is hidden from the general public, often referring to subcultures or movements.
Example: "The city has a vibrant art scene that exists as a sort of underground, with many artists showcasing their work in hidden galleries."
Alternatives: "a kind of subculture" or "a type of hidden scene".
She attended a school called the Flying University, which was a sort of underground university since women weren't able to study in her era.
The broadcaster's role as a multicultural hub where cult followings begin before going mainstream, gave the contest a sort of underground feel to it.
Every stop on what has become a sort of underground railway is shrouded in secrecy: requesting supplies, confirming the needs, delivering goods, fund-raising and collecting donations are carried out by separate cells.
A story spread on the internet claims that thousands of Hamas fighters dressed in Israeli army uniforms had been planning to surge through the tunnels into Israel in a sort of underground D-Day landing.
The generations of girls who have inhabited the White House compose a sort of underground society, initiating one another into the place's charms — "Have a helluva good time," Alice Roosevelt Longworth wrote to Susan Ford.
At first it was a sort of underground stop for food and wine lovers who had heard word of small, fascinating restaurants run by young, talented chefs serving a bounty of local produce.
Although Linux began as a sort-of underground movement in response to the dominance of Microsoft's msft proprietary code, the Expo shows how potent a force the operating system has become in the computer industry.
Every new gadget seems to generate a sort of hobbyist underground, a collection of people who want to disassemble its hardware and decode its software.
And if only they could find a few scientists willing to report that it's not the calories but the lack of exercise that's making people obese, then they can use this as a sort of an underground marketing strategy.
It continued into the 1940s and has survived since then in a sort of Christmas Underground made up of traditionalists who remember it from their youth and expats from Germany and the Nordic countries who brought the practice with them to the U.S. and have never given it up.
Instead, she became an actress, parlaying her Chelsea Girls greatness into work for Roger Corman ("the only time I saw him, he told Sylvester Stallone this was a 'tits and ass movie', so he'd better uncover his ass, which he did") and all the maverick young directors under him in 70s LA - a different sort of underground.
