Crossword Genius

Presumably it will show up lack of weight (5)

Ross

I believe the answer is:

light

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'weight' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's a link between them I don't understand?

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

Can you help me to learn more?

(Other definitions for light that I've seen before include "Torch" , "Illumination weighs very little" , "Illuminate or ignite" , "Whitish" , "dawn" .)

I've seen this clue in The Irish News.
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