What is a clear jel substitute?

There is no good substitute for Clear Jel as is it used in home canning of pie fillings, according to Pick Your Own. The gel is a modified corn starch that does not break down when exposed to food acids and doesn't interfere with heat sterilization of potential bacteria in canning.

It is not possible to use another thickener when canning pie fillings and other thickened fruit desserts because they tend to break down in the canning process. However, cooks can make up their pie fillings and then can them without the thickener. Just add a traditional thickener, such as cornstarch or flour, to the filling before cooking it in a pie.