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 they are linked in a way I don't understand?
'border' is the second definition.
(I know that edging is a type 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" .)