It would get the morning sun then.You would need quite a large pot and as clematis like roots in shade and heads in sun you would need to shade the base of the plant either by having another pot of something bushy next to it or by a thick layer of gravel or stones on the top. A bushy plant would be best as it would also keep the pot from heating up in the sun. If you look up clematis on the A_Z at the bottom of this page you'll find loads of information. I'm sure you will get more detailed advice from other members before long!

What you have to take into account is that, it will only get a little sun in the morning, the rest of the day in possible shade, also a strong biting east wind in the winter can be very damaging, now i have seen clematis montana planted in this aspect and in some cases do quite well, but these have been ground planted, not in pots, and in the recent cold winter in my jobs i have noted that some montanas have been killed off, mainly the top growth, but lower down new growth emerging, you could opt for parthenicus,[viginia creeper which would be ok in this aspect, very quick growing and would quickly clothe the wall for you, and the foliage in the autumn is well worth the wait, the wall will look has though its on fire, julien.

I have 3 in an east facing border, Comptesse de Bouchard, The Princess of Wales and Belle of Woking and they all do well, these are all in the ground though.

Your biggest challenge will be finding a suitable pot because most will be far too small - these plants have long roots.
It'll need to be about 2' deep and about the same diameter.

You'll also need to provide a thin metal obelisk for it to twine its tendrils around because these plants need a structure to climb around - made of thin material because those tendrils are only about an inch in length and need to wind around its host.

Sorry to disagree with you Pennyfarthing. Planting a bushy plant in a pot alongside a clematis is the last thing you should do. Clematis need to have their feet in a moist soil and will need to be kept watered. In a pot with another plant fighting for the available moisture will probably end in tears for the clematis. Is there even a possibility that you could plant in the ground Deb. Welcome to GOY I'm sure you will get lots of good advice.

First, in an east facing position, you don't need to worry about shading the pot or roots too much, unless you know it gets very hot there, but you can get round that simply by standing a smaller pot with something in it in front of the clematis pot. Second, choose a smaller hybrid clematis, not a species (montana is a species). As Louise says, a pot 2 feet deep is best, but it only needs to be a minimum of 12 inches across, more if you can find it. You will need to put up some support for the clematis - the simplest is probably Clematis mesh, which is plastic coated wire - comes in green, white or brown.
The next thing is to decide when you want it in flower - I'd recommend something from the spring and summer flowering group, because most of those will grow in any aspect, so some of the smaller ones in this group would be Lady Northcliffe; John Warren; Niobe; The President; Violet Charm. If you prefer one from the summer and autumn flowering group (these need pruning down to about 2 or 3 buds by mid February every year), then Comtesse de Bouchaud, Hagley Hybrid or Rouge Cardinal would be suitable.

Sorry Scotsgran-I have misled you--I didn't mean to put a bush in the same pot but in another alongside. However, it seems most people think the sun is not likely to cause problems in this position.

My mistake Pennyfarthing I should learn to read for effect. I made an assumption and having read your answer again I can see you did not mean to put the bush in the same pot. Silly me.