There is some sort of a compromise going on here. This could be a residue from an old abcess growing out BUT there is some other seperation happeneing. Trimming is due but I would not be suprised if there isnt some white line disease starting there. I wuld get a very knowlegeble Farrier in there to take a gander or a Vet to see what exactly going on there. In the mean time I recommend Sole Pack Poulticing patties (sold seperatly or in bulk). This stuff is great for removing sorness and for thrush (you can shove it in to the clefts of the frog if need to) and for abcess and for bruisess and everything in between. The poultice is a putty consistancy and can be applied very locally or you can apply it generaly to the entire underside of the hoof. I have used it many times to pack in old abcess blowouts and breaching in the white line due to abcesses that are growing out. (such abcesses don't nessesarily cause pain and escape from the coronet band then grow out towards the exposed undersurface of the hoof. These types can indeed lead to other issues due to the breached area between the lamina layers (the sensitive and insensitive layers). Its a place fungus can go nuts esp if large enough when growing out towards the exposed undersurface. .I have also used sole pack for thrush (in a compromised hoof also) with great sucess. (Sole Pack also comes in liquid form.)
I have also packed with sugar dine soaked cotton pushed into the hole in question. (sugar dine is sugar mixed with tincture iodine.) it should be mixed thick and balls of cotton soaked in it. Wrap it up and change in 24 hours until Vet or Farrier can come out and take a closer gander at whats going on.
"The question is not, can they reason? nor, can they talk? but, can they suffer?" Jeremy Bentham