I cannot say what might causes an individual of one species to see an individual of any other species as being a rival. But it happens. Being relatively close in size and/or having similar dietary preferences is often all it takes. Some individuals will never change, even when their opponents essentially surrender/defer.
Breaking up lines of site is one way to help handle this sort of thing. More stuff on the substrate would be applicable here. The other option you cannot control, and that is for the plecos which are being attacked to become more adept at staying out of sight of the phantom better. More than just hidey holes may be needed. The clown may need ways to move about under cover.
There must be a reason the phantom chose the center of the tank. They do like high current bud do not have to have it. So that may be where on the bottom of the tank it is strongest even if that is not much. Or it just might be because how the tank is scaped that makes it the best spot in the mind of a pleco
So, one thing to try is to change the aquascaping and the flow pattern. Hopefully, this will cause the phantom to pick a new spot more towards an end of the tank. Then you can do a bit of rescaping so when in its area the phantom cannot easily see more than a third of the way across the rest of the tank. When you feed make sure the phantom does not have to go far from "home" to find food.
Another help for the clown is to create a space where the clown can move about some under cover. The clown needs to be able to run for cover and, once there, move someplace where it can get in or though but the phantom cannot. This will get easier to create the bigger the phantom gets than the clown.
If the corys are not being harmed but chased off, the above set-up should minimize that some too. But corys are usually curious and all over the place. If they are being harmed they should learn that they need to avoid the phantom and his territiory.
Of course there is always the option of last resort. You can rehome either the phantom, and get a new one or get a different species that will live with the clown. Or else to can replace the clown with a different species that cannot be bullied by the phantom and wont bother it unless the phantom gets stupid.
To paraphrase Yogi Bear, remember, "You are smarter than the average fish."