NPR: Mark Zaid explains that DT could even give sensitive intelligence to Putin if he wanted to.

https://www.npr.org/2019/03/01/699407475/what-you-need-to-know-about-security-clearances-inside-and-outside-the-white-hou?utm_source=twitter.com&utm_medium=social&utm_campaign=npr&utm_term=nprnews&utm_content=20190302

Does Trump have the power to overrule them?

Yes. It is an executive branch function, and as chief executive the president has the ultimate authority.

Not only did President Trump have authority to give his son in law and adviser Kushner clearance, the president "could conceivably give Vladimir Putin a security clearance," says Mark Zaid, a Washington attorney who specializes in national security cases.

He said the president can furthermore declassify "the family jewels" — meaning secret documents — "and give them to Putin," if he wanted to.

Unlikely as that may seem, Zaid he could find no other occasions where a president has actively involved himself in granting, or withdrawing a security clearance.