Almost certainly slugs and snails. I pruned one of of clematis today and it was full of snails happily chomping away.

My problem was earwigs on the clematis. Go out after darkness falls ,with the torch, and in the blooms you will find 3/4 earwigs chomping away. I would think I have caught over 100 earwigs in the last 10 days.I also use upturned pots full of straw, very successful also.

Earwigs on clematis - that's a new one!