It sounds like you have a fungal disease like leaf spot.  Start spraying with Bonide fungonil right away asnd follow the spraying schedule on the product’s label.

To avoid fungal diseases, NEVER spray your plants with water.  When you need to water, direct the water onto the soil below to water the roots while keeping the leaves of your plants (not just the clematis)  as dry as possible.  With the hot, humid weather and intermittent heavy rain we’re getting, we’re seeing a lot of fungal diseases cropping up.  No need to add to the problem by spray watering.