I have had some experience with Zebrasoma's and aggression. I too have the
combo you mention but I have no issues.

I am guessing the size of the flame angel and the Yellow are similar. If so
I am also guessing the yellow was the last one in the tank.

If you do not see thing settle down soon (like a day) it will only get
worse. Find a way to reintroduce the flame angel to the tank. One way to do
this is move it to a quarantine tank for 2-3 days. When you put it back int
he main tank it will be less agressive.

If you do not have a quarantine then hopefully you can swap it with another
at your LFS. Either will work.