Leanne, it's hard to say just how the shoot will behave, but if it's new growth from a good rootstock it may bear decent fruit in a few years. A lot of pears are grafted to one of a series of rootstocks that were developed following the destruction of many major pear orchards by fireblight decades ago. But they may also be on seedling rootstocks, which simply aren't as defineable and predictable as the Old Home /Farmingdale crosses. But many are hardy and perfectly good choices.

As to how long your tree will stand? Quite a few years, I should think. I doubt that whatever killed your pear will affect the clematis. It could be a very attractive arrangement.

Good luck,

Mark

Here is a link that might be useful: Romeo & Juliet of pears

Dont cut off the shoot to repot it - pears dont grow that way. It could bear some edible fruit. It could also serve as a new trunk that you could graft a better type of pear to.