Making slow progress with border? (6)

I believe the answer is:

edging

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

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for edging that I've seen before include "Border, fringe" , "Moving slowly sideways" , "Border, fringing" , "Siding; border" , "Making a border" .)

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