Should President Biden Deny Donald Trump Access to Classified Material Beginning on January 20, 2021?
A sitting president doesn't have a security clearance per se. It's automatic.
But once leaving office, he/she doesn't really have one.
However, access to classified information can be granted to former presidents and former vice presidents.
U.S. Code:
https://www.law.cornell.edu/cfr/text/31/2.2