KH, like most hornbeams, is VERY slow at closing large pruning wounds. The one on your tree was probably made when the plant was originally cut down into a stump from a larger twin trunked tree.
It has not really attempted to close in all that time. If this is an import, that scar is a likely reason is was shipped out...
Anyway, the tree is beautiful anyway, just will take a bit of work to close up the wound.
The fastest and most effective way to do that would be to carve out the wound, and "re-injure" the tissue around the wound with a sharp carving blade, or even a light treatment with concave cutters.
This will re-stimulate the callusing process. To speed up callusing, transfer to a larger pot to allow more root run. Allow it to grow unpruned for a couple of years. Planting it in the ground will accellerate things even more. If a new shoot or two develops at or near the wound, let it grow out as much as possible. It will further stimulate healing in the area.
Of course, if you allow the plant to grow unrestricted for a few years, you will lose the branching as it goes wild. That's really not a big deal, as KH responds very aggressively to extremely hard pruning. Cutting it back to old wood usually produces an explosion of new shoots that can be developed using the cut and grow method.
So, bottom line, if you want to heal the wound, it will take probably five years or so to close the wound and redevelop the branching.
All that said, the tree is beautiful as it is...