Panicum virgatum, commonly known as switchgrass. Clethodim will kill it, I'd probably mow it and then spray it. 16 oz pr acre of roundup will help the process without killing the clover.
You may want to consider lowering your carbon/ nitrogen ratio in the soil. Longterm clover plots raise the nitrogen to the point that the clover has limited carbon food and the grass has a smorgasbord of nitrogen food, therefore the grass can almost not be controlled anymore. Interseeding grains into the clover in early spring or early fall helps achieve this, especially rye in the fall, or oats in the spring. The decaying rye straw will be a huge benefit to the longterm soil health in your clover plots, and rye’s weed-management benefits would help with your switchgrass control, you can allow its allelopathic effects to persist longer by leaving killed residue on the surface rather than incorporating it.. Rye shouldn't be spring seeded because it needs to vernalize (be exposed to extended cold after germination), oats and barley do better than rye in hot weather.
This is the one thing annual ryegrass is acceptable at. White clover competes well with it, it gets rid of nitrogen, and dies in early summer making a nice mulch. Here’s a picture of mine I made today.