What's light to carry (7)

I believe the answer is:

lantern

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'what's light' is the definition.
(lantern is a type of light)

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

(Other definitions for lantern that I've seen before include "Enclosed lamp" , "Leading light" , "Portable lamp case" , "Encased light" , "Light held by 26/27 Across" .)

I've seen this clue in The Sun.

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