By Abertawe
United Kingdom 
I notice my clematis 'avalanche' which flowers in the spring is covered in a brown scale like spot on the back of the leaf.
It is in a very warm, sunny spot and wondered if I should move it. I have never come across anything like this before.
13 Jul, 2020
Sounds like scale - check the stems too. Scale is more likely if, as you say, your clematis is in a hot, sunny spot, further info below, scroll down to scale - there's an image of one attached to a stem
http://www.britishclematis.org.uk/pests.htm
13 Jul, 2020