Crossword Genius

Energy with which an opening may be closed (3)

Ross

I believe the answer is:

zip

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'energy' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

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(Other definitions for zip that I've seen before include "Energy (informal); a US code" , "Fastening device" , "Sound of bullet passing" , "tear" , "drive" .)

I've seen this clue in The Globe and Mail.
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