Have you seen any earwigs, I know they like to munch on clematis leaves and flowers. Or could be cutworms, they come out at night and devour the leaves, then hide in the soil during the day. You could go out after dark with a flashlight to see what is eating them. Most bugs eat at night so as not to be eaten by the birds.

I had to look up cutworms, they are a moth caterpillar [yellow underwing in my case] and I find them a lot in the garden, munching on a range of plants.

thanks teds, sbgirl i did think of putting grease of some sort on the stems but dont know if it will harm the plant

I wonder if its caused by a leaf cutting bee - they do take sections out of leaves rather like this. I was once lucky enough to see one carrying a bit of leaf away with him!

thanks steragram i,ve heard of them never seen one do they look like a bee or a wasp, because i saw what looked like a wasp near it but i thought wasps came out abought september?