With such a minimal description of the piece of furniture, my thought is to remove the screw and, assuming that the referenced split is to the outside of the leg, try to work glue into the crack as much as possible. Draw the split together with a wood clamp and set it aside to dry overnight. Then drill the screw hole to tightly accomodate a short length of hardwood dowel. Once you have glued in the dowell, drill it for the proper size screw and rescrew the leg.