First off, if possible, wet rehydrate the area, you are repairing, as well as some of the area beyond this. If you get the cracked area wet enough, you could possibly just use some standard slip, with a little scoring to pull the clay particles back together.
You could also use paper clay, in combination with the above method, which will be less likely to shrink, when it dries, minimizing the chance for it to crack again.
However you fix it, let it dry slowly again. An area that is rewet, the repaired, will crack again if dried quickly, sometimes even worse than before.
So the good news is that it can be repaired. The bad news is that no matter how well you do it, it might reappear during the bisque or glaze firing, especially if you fire high.
Cracks can be from structural issues, and just don't go away. It's like an old house, where you keep patching the same spot of cracked plaster. It's not the surface that's the problem, it's what is underneath.