Gradually moving border (6)
I believe the answer is:
edging
This might be a double definition.
'gradually moving' is the first definition.
The answer and definition can be both to do with motion as well as being present participle verbs.
Maybe you can see a link between them that I don't see?
'border' is the second definition.
(edging is a kind of border)
Can you help me to learn more?
(Other definitions for edging that I've seen before include "Siding; border" , "Advancing sideways" , "Making a border" , "Bordering or on the fringe" , "Sidling" .)