Pollard closely packed trees to expose borders (5)

I believe the answer is:

edges

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'borders' is the definition.
(an edge is a border or margin)

I cannot understand how the rest of the clue works.

(Other definitions for edges that I've seen before include "Fringes" , "Bounds" , "Outside limits" , "They're used for cutting" , "Margins" .)

I've seen this clue in The Guardian.

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