It is normal for plants to shed leaves as they age, particularly those at the base of the plant that are starved of light. However, I notice that only some of the leaves are affected - have you checked whether this problem affects all of the stems or just a single stem? If it is just a single stem then you will need to investigate it much closer to determine what the cause of the problem is (mechanical damage, pest damage, virus etc.) It may need to be pruned out entirely. Clematis can form quite large root systems and will use a surprising amount of water so it is always beneficial to give them plenty of room to grow. If the plant is at all root bound then you will need to repot it in fresh compost. Regular feeding and watering will also help to promote healthy new growth, but take care not to over water as they dislike a waterlogged soil.