Answer: If your soil is highly alkaline, copper may help prevent chlorosis. But what you describe doesn't sound like chlorosis (where the veins remain deep green but the rest of the leaf looks pale green to almost white). You might want to test the soil's pH so you know what you're working with.
Clematis grow and flower best in a good neutral soil. If your soil is acidic, you should definitely add lime which will make the existing nutrients in the soil available to the plant. If your soil is alkaline, you'll want to add iron (ironite is a good product) along with your fertilizer.
While you're waiting for a soil test, it wouldn't hurt to feed your clematis with a water soluable fertilizer such as Miracle-Gro or Peter's. Mix a half-strength solution and apply every 2 weeks throughout the growing season. Mix 1 TBS per gallon of water and pour on the soil around the plant. Depending upon the size of your plant, you may need to use 2-3 gallons of the mixture each time you water. The liquid fertilizer can also be sprayed on the foliage, where it will be absorbed by the leaves. This should help it green up.
Best wishes with your clematis!
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