This might be helpful
Thickener:
Instant Pudding
Add a package of instant pudding to the filling mixture. This will actually improve the flavor of your pie by giving complexity and the gelatin in the pudding will help the filling set up. A good combination is lemon with fruit fillings.
Tapioca
Use tapioca flour, not granules to thicken things up. The tapioca granules take a while to fully hydrate and you could wind up with weird grit in your pie and have the filling still be runny. Instead use tapioca flour or use a blender to pulverize tapioca granules. Read the back of the package to find out how much you should add per cup of fruit. Mix the tapioca with the other filling ingredients and allow it to sit for 15 minutes to dissolve the tapioca before you put the filling in the pie crust.
Corn Starch
You can add corn starch to your pie filling. About 1 teaspoon of cornstarch per cup of filling. 9 inch pies should hold 6 cups of filling. Cornstarch has problems when used in very acidic pies. So try a different option if you are working with citrus.
Flour
I dislike adding flour because it makes the filling cloudy and does nothing for the flavor. However, sometimes it is all you have on hand. Use 2 teaspoons per cup of filling to thicken up your pie. Make sure you use a sieve to sprinkle the flour into the filling or when you mix it you will get clumps of flour. Dumplings don't belong in pie.