Border gradually moving (6)

I believe the answer is:

edging

I'm a little stuck... Click here to teach me more about this clue!

This clue might be a double definition.

'border' is the first definition.
(I know that edging is a type of border)

'gradually moving' is the second 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?

Can you help me to learn more?

(Other definitions for edging that I've seen before include "Border; moving carefully" , "Sidling" , "Advancing sideways" , "Inching sideways" , "Siding; border" .)

I've seen this clue in The Guardian.

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