Knowledge having little weight (5)

I believe the answer is:

light

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'knowledge' is the definition.
(light is a kind of knowledge)

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

(Other definitions for light that I've seen before include "Airy; not heavy" , "Illumination weighs very little" , "Person of authority" , "Not dark; not heavy" , "Beam" .)

I've seen this clue in The Irish News.

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