Thank you for writing.
This is White Snakeroot. https://www.minnesotawildflowers.info/flower/white-snakeroot
It is poisonous to animals that ingest it.  It spreads far and wide and should be dug out and bagged and not composted. Alternatively you can use weedkillers as directed taking care not to have them drift to other valuable plants. 
https://extension.umn.edu/horse-pastures-and-facilities/identifying-and-controlling-weeds

Thank you for writing.  The roots of clematis are extensive so it will be hard to wash them for separation from the snakeroot.  https://www.hummingbirdfarm.net/transplanting_clematis.htm The washing will increase the transplant shock. Applying a herbicide to the snakeroot may poison the clematis roots.  I suggest that you keep cutting the snake root right at the ground all summer to prevent it from seeding or feeding the roots.  You can try digging up the planting in Sept and separating them.  But sometimes you just have to buy a new plant.