Interveinal yellowing of leaves is usually an indication of iron deficiency. It might be due to a pH problem. If the soil pH is high in the root zone of the clematis, it can't take up iron properly. To get a quick reversal of the coloration you could apply iron chelate to the plant. In the long-term, you might also want to amend the soil with elemental sulphur to lower the pH. The soil next to concrete foundations is often higher in pH than the rest of the yard because of lime leaching from the concrete.