Certified GKH Gardening Expert
You can cut these back, but it is best to know when they flower, since there are multiple types of clematis. You will prune them during different times depending on their type.
Pruning out of season will damage the next years flowering production, so if you are unsure, then it will be best to leave the growth until you can see what is still alive and what is dead.
This article will help: https://www.gardeningknowhow.com/ornamental/vines/clematis/pruning-clematis-vines.htm