Abbiejan, you will have to provide some sort of support for a clematis to climb any slick surface. Some people use netting of one type or another. If you have green vinyl coated fencing, you can wrap it around the lamp post to get the clematis to climb that. Just leave enough space between the support and the post to allow the clematis vines to twine around.

I just bought this trellis. I am going to use it's parts separately, like on the picture, but all 3 parts together make a circle for the planting around a lamppost or a tree.

I used some 1"x2" wire fencing to make hoops to surround the pillars of my pergola. The clematis have gone all the way up easily without help.

i have used wire in a few spots to get things to climb and it works well without being very noticeable. i use a somewhat thin gauge, but still heavy enough to have strength to stand away from the post or pole i'm trying to cover. i've done this on my lampost to get english ivy to hold, and then i have an alba luxurians clematis growing up the ivy. i was actually able to remove the wire this spring, after 2 years. i've also used it to get morning glories to climb the ugly electric service pipe on the back of our house - works like a charm. have fun with your project!

Good old chicken wire is great. I cut a piece long enough to loosely go around the post and tie it off with "twisties" from bread bags or such. It is very inexpensive and you can buy short rolls at Lowes or Home Depot. You can place one piece on top of the other if you want it to go real tall up the post. You really never see the wire once the plant fills in.

The only problem with the trellis you bought, the spacing may be too far apart so you may still have to fill in with wire or something. Good luck.