Is this a thickwall or thinwall type pipe? 1 1/2, 2 or 4 inch?
Anyway...there are various things you can do.
You can glue and tape it. Use Goop household adhesive sold in the glue aisle of home centers. First roughen up and clean the surface. Coat the crack area and extend out beyond. Then after it dries enough, then apply a 2nd and even 3rd coat to give it some thickness, as on a vertical surface, it will get thin and have a tendency to run. Then, after these coats are dry enough, tightly wind electrical tape around it. This is one way.
Another way is to section out the 6 inches. Then get one or two Fernco (type) couplings and a new aprox. 6 inch piece of pipe. Then couple in the new section. The fernco couplings are thick tubes of rubber that you tighten with stainless steel bandclamps that are already included on them.
Or you could do this with one Fernco coupling on one end and a pipe coupling glued on the pipe on the other end.
Or, if the pipe has give to it lengthwise...at least in one direction, when you cut it... you could use a glued in coupling on each end with a replacement 6 inch piece of pipe in between. This is the way I would make the repair if this is possible.
Trying to clamp rubber successfully over such a length of a crack I don't think would be a good idea, as I doubt you would be sussessful....even tho' this is a common method used for small holes or small cracks in pipe. I have done these type repairs and they actually sell the correct size diameter rubber-fitted clamps for doing so. But for a 6 inch crack?....No, I don't think so.
That Goop adhesive is some amazing stuff. I call it the lazy man's epoxy, as it is about as good as epoxy, except you don't have to mix it. I have used it to repair a worn hole in an otherwise god $35 washing machine discharge hose, and sealing rusted drain line joints where the water was leaking out like a seive from the stories up above, in galvanized metal and cast iron, and have made these repairs years ago and never had a call back. I've use the stuff, for a kinds of things...like adding it to the perimeter of the joints of dresser drawers that kids broke in pieces, to lock all the parts together. Once you use that stuff you will get hooked on it. It sticks to virtually anything, also. It's great to keep this Goop, electrical, and duct tape on hand.