Dave Meltzer in the Wrestling Observer Newsletter confirmed that it will indeed be Seth Rollins defending the title against Braun Strowman.
The storyline explanation on why Rollins is getting a fresh opponent is that Brock Lesnar is not being granted an automatic rematch.
The general rule for Clash Of Champions is for all titles to be defended. It looks like Kofi Kingston will defend the WWE Championship again against Randy Orton and Becky Lynch will defend the Raw Women’s Championship against Sasha Banks.
A match that was teased for SummerSlam that didn’t happen was Shinsuke Nakamura defending the Intercontinental Championship against Ali. It looks like that match will happen at Clash Of Champions.
It also looks like they are setting up for Big E and Xaver Woods defending the Raw Tag Team Championship against The Revival. The finals of the King Of The Ring tournament will take place on that show.