That will depend on the type of Clematis. Try googling 'pruning clematis' to find out which type needs pruning and when. Some don't need it, so it's best to know which is which.

Hi Pilly and welcome to GOY. there are 3 specific pruning groups for Clematis and you need to match it up with your plant so next year you get flowers . It can be very confusing! Spritz a member has written a good blog on this subject also if you enter clematis pruning in Goy search box (top left) there is a lot of discussion by members to be found.
If you go to the bottom of this page and click on C you can select a page thats has lots of members Clematis plants for you to see.

If it is a specie that flowers in early spring do not prune it. Only light trimming to keep it in shape. If it is a specie that flowers in the summer cut it down to the base in the autumn.