A couple of years ago, i bought a clematis in the fall and had nowhere to put it. I buried the container in the vegetable bed, piled some leaves on top, dug it out in the spring and planted the clematis. It grew and bloomed. I would say to do that rather than try to overwinter it in a container above ground.

I agree with Marcia, that you wouldn't want to leave the pot above ground for the winter. Even in an unheated garage I think it would be risky as it would probably freeze solid. As for planting it in the fall out of a container, I think it would do fine as long as you don't disturb the root ball too much and could give it a good month to settle in before freeze up.
Good luck!
Laurie

The last few years one of our local garden centers has clematis on year end clearance....around 25cents/gallon pots. I've bought a couple hundred and 95% overwintered fine in the pots just by covering them with leaves. What I also like is that they 'take off' earlier the following year than those planted in the ground.

We've also have viticellas in large square pots next to the house and they overwinter fine. One side is next to the house for warmth throughout the winter. These clematis also 'take off' before their cousins in the ground.

Alpiner those are my favorite kinds of clematis...the ones that are on clearance or in the left for dead pile! Back in Halifax, I got a bunch that were crispy and dead looking for free, took them home soaked the dried out pots, re-potted them and they grew like mad. People don't realize that the most important part of the clematis is the root.
I would go with Marcia's method, it would give them a bit more protection.