Cut the rhubarb into small pieces. Think small than one 1 inch.
Mix together the cut rhubarb with the white sugar, little bit of flour, and cinnamon. Dump it in your slow cooker.
Mix together the remaining, creating the crumble. Spread the crumble over the top.
Place lid and cook on low for 3 hours.
If you are using a small (2.5 qt) you’ll need to cook it an hour longer. If your are using a larger, casserole sized one then the 3 hours on low will do the job.