If the particle board behind that ugly area of veneer is soft, drill a few holes in it and inject that thin, flowing epoxy called Git Rot. That will bond with what's left in there, then clamp it closed with a board on top of it to make it flush with the rest of the panel. You could do the same with that separated seam, clean out the loose stuff, inject it, clamp it closed. I was able to repair a corner of an old Jensen cabinet with it, it worked well.