Container Clematis Winter Pruning
Here is information on pruning your clematis. Look at Group 3 which are pruned in the spring. http://web.extension.illinois.edu/cfiv/homeowners/110414.html
To overwinter outdoors Plants in containers need to be 2 zones hardier than your zone. You may be able to place the container next to your house and cover with pine boughs or shredded leaves for winter protection. Or sink the container in the ground to protect the root zone from the cold temperatures.