One through to slips or boundary? (4)

I believe the answer is:

edge

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'boundary?' is the definition.
(I know that edge is a type of boundary)

I don't understand the rest of the clue.

(Other definitions for edge that I've seen before include "Skirt" , "Border; intense quality" , "Snick - boundary" , "marginal advantage" , "Side, brink" .)

I've seen this clue in The Sun.

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