It's a group 3, apparently. That means you can prune it back hard in March, give it a feed and a drink and it will grow new stems you can train in as you see fit. You can do this every year. http://www.clematis.hull.ac.uk/new-clemdetail.cfm?dbkey=557
For now, you can just prune it back to reduce its size if it's swamping other plants and then, to make it easier next spring, you could reduce the whole thing back to a couple of metres high and wide in late autumn and pull out all the top growth after giving it a week or two to wilt so it doesn't damage any plant it is swamping but you have enough growth left to take any severe winter frosts and thus protect the crown of the plant.
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For now, you can just prune it back to reduce its size if it's swamping other plants and then, to make it easier next spring, you could reduce the whole thing back to a couple of metres high and wide in late autumn and pull out all the top growth after giving it a week or two to wilt so it doesn't damage any plant it is swamping but you have enough growth left to take any severe winter frosts and thus protect the crown of the plant.