Even in a pot - zone 6a? I have the perfect pot/trellis combo that I want to plant clematis in. I have to wait until my Roma tomato plant kicks it at the end of this season. I was thinking forward to planting something new in it next spring.
You could simply sink the pot in the ground to overwinter it outdoors... if that's even necessary in your climate.
For successful overwintering in a dormant state indoors, you need a cold room, i.e. a basement room that can be kept at slightly above freezing (not room temperature) and totally dark. Otherwise, the plant will not stay dormant.
Should be fine--wouldn't hurt to doublecheck your particular variety, but many of them are hardy a couple zones colder than you. So while being in a pot can knock a bit off the hardiness, you're far enough above the cold end of their range that it should be OK.
I agree with the others and would say your Clematis ahould be fine in a pot over the winter in your zone, The winter weather here in Scotland can be anything from zone 5 - 3 but NOT for very long periods.
All I would add is if pos, repot NOW, into a larger pot so you can sink the plant deeper under the soil in larger pot and top dress with more compost, IF you dont get ground freeze in your area just top dress the pot with leaf mold for winter to help preven frost in the evenings, or as sugested, sink the larger pot into the garden maybe in a more sheltered area and bring the plant and pot out, come early spring,
When you do plant this clematis out in the garden or realy large pot with a trelis, add plenty compost / manure to the soil as you plant, they like a good rich soil, shaded or cool root area, but top /foliage in sunshine on a frame or wires to climb and to achieve this, after you plant the Claematis and fill in the hole you can cover the root area with plaenty of leaf mold or even a flat larger stone Paver or similar.
make sure you water after planting out in the garden and give a sprind dressing of plant food and rake this in.
Make sure you choose the right spot as once established, lot's of Clematis dont like to be disturbed by transplanting.
Good luck. Weenel.