This was a crack in a hoof that went from the bottom of the hoof to the top of the coronary band; which was causing slight lameness. Left untreated this would lead to serious lameness and is not something that heals by itself.
A full diagnosis enabled me to attach an aluminium plate (in this case it did not need gluing) to hold the two parts together so that the pressure was relieved from the coronary band so that it could heal.