“Here, snag my hand!” Alai called.

Ender held out his hand. Alai took the shock of impact and helped Ender make a fairly gentle landing against the wall.

“That’s good,” Ender said. “We ought to practice that kind of thing.”

“That’s what I thought, only everybody’s turning to butter out there,” Alai said. “What happens if we get out there together? We should be able to shove each other in opposite directions.”

“Yeah.”

“OK?”

It was an admission that all might not be right between them.

(Source: this ELL question)

What is the grammatical role of the that-clause here?

I first thought it's a noun clause serving as the subject of the sentence:

That all might not be right between them | was | an admission.

But such reading strips the sentence of its original meaning.

And is this really a noun clause?