Presumably it will show up lack of weight (5)
I believe the answer is:
light
'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" .)