Advantage in bushy boundary having top trimmed (4)

I believe the answer is:

edge

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'advantage' is the definition.
(having the edge is having an advantage)

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

(Other definitions for edge that I've seen before include "Sharpened side of blade" , "Creep" , "Golden ingot" , "Keenness" , "Bind" .)

I've seen this clue in The Sun.

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