Why don't you try invisible black netting? You drape the netting round the post. The clematis tendrils will grab it and then it is up, up and away. You netting is not really visible unless the lamppost is white..... is it? You can buy the netting in any hardware store.

I have cut widths of chicken wire to support vines around metal posts. It usually gets covered enough in vine but you could spray paint if you want before wrapping it around post.

I have used the black plastic netting that Debra suggested. You can find it at Lowes or Home Depot and is the stuff they sell to keep birds out of fruit trees. It is relatively inexpensive and a roll will last a long time if you have to replace it each year.

Another option is dark green vinyl coated wire fencing. The rolls are typically 4 feet wide and if you need something taller, you can wire two pieces together to get up to an 8 feet length. It also is available at Lowes or Home Depot.

I tie (or use screw or some type of fastener) clear fishing line at top of lamppost (or whatever structure I want to cover) and then just coil it around a little loosely down to base where it is secured..using screw or staple, etc. You can coil the line close together or further apart. It can remain up yearround as it is almost invisible.

This works well for Japanese Climbing Fern or Gloriosa Climbing Lilies once you've helped them make contact with the fishline. Once in awhile you might have to corral a wayward tendril and guide to line.

I've just grown a couple of the small-flowered species Clematis thru shrubs but I don't see why this wouldn't work for them, too. If you're growing the large-flowered cultivars you might want to be sure to get the heavier weight fishing line. Not sure what I bought but I know there were several thicknesses available. josh