Clematis Jackmanii is a group 3 Clematis. You should prune it back to the first pair of leaves/buds EVERY year. Late winter or early spring is a good time to do this in your zone.
It won't be too late to prune it now this year. Cut it back and fertilize it. When new shoots appear, pinch them to stimulate branching.

Thanks, but ... the vine is just one old stem that grows up to the top of the 6-8 ft. fence. The only leaves/flowers are right at the top in a bushy-like form so nothing below that to prune. If I prune below that bushy top on th bare stem I'm afraid I will kill it? I just bought a wilted plant on sale years ago & having had no prior experience with clematis, I never expected it to survive!

You will not kill it. In fact, you will help it to get established. Young plants should concentrate on growing roots, not vines. Your Clematis will send 2 new shoots from the bud union just below the cut. Pinch these shoots when they will be several inches long. Do not forget to fertilize the plant right after cutting it back.

Thanks, alina! I shall wait til it goes dormant down here & do what you said. That tall spindly thing looks so unhappy!!

Jackmannii is a Type III hard prune every year.

You can cut it back now. You do not have to wait until it goes dormant.

You should cut it again next and every early spring/late winter.

Pruning helps Clematis, it does not hurt them.