... was a belief that racism is no longer a systemic problem

[Ostap]

In a study of the 2016 election published last year, the Tufts University political scientist Brian Schaffner and two of his colleagues found that the strongest predictor of support for Trump over Hillary Clinton was a belief that racism is no longer a systemic problem.
The Atlantic

Hello, teachers. I know that "the" is more common in such cases, because the defining relative clause is used ("that racism is no longer a systemic problem"), and that's logical. But what justifies using "a" in such cases?

 

[Skrej]

It's only one of many possible beliefs.

 

[Ostap]

It's only one of many possible beliefs.

One of many beliefs (in general), or one of many beliefs that racism is no longer a systemic problem?

 

[Skrej]

One of many beliefs, or at least beliefs related to supporting a particular candidate. I suppose you could narrow it down to beliefs about racism if you wanted to.

 

[Tarheel]

Systemic racism would have to do with laws, as with Apartheid or Jim Crow.

 

[jutfrank]

a belief that racism is no longer a systemic problem

The bolded part is an indefinite noun phrase. The unbolded part expresses what is believed. It isn't a relative clause. You can see this more clearly if you do a simple transformation:

They believe that racism is no longer a systemic problem.