It depends which Clematis it is, some can be cut back hard and some not so hard

Only prune it at this time of year if it's a Clematis montana or another winter-flowering plant, such as a C. armandii or a C. cirrhosa. Most other summer-flowering Clematis should be pruned in the early spring. Group 2 Clematis only need a light trim, group 3 should be cut back to healthy buds about a foot to 18" up from the base.

So Lulu is right - you need to know which clematis it is.

Hows about a photo