Is such reading illuminating? (5)

I believe the answer is:

light

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'illuminating?' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for light that I've seen before include "Visible rays" , "Beam" , "Not too demanding" , "Pale" , "Hint, clue" .)

I've seen this clue in the Evening Standard.

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