Crossword Genius

Darkness arising when there's no good light (4)

Ross

I believe the answer is:

thin

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'light' is the definition.
(similar in meaning)

I cannot understand how the rest of the clue works.

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(Other definitions for thin that I've seen before include "Insubstantial" , "Watered down" , "Measure" , "Lean; make (plants) less crowded" , "Slim - watery" .)

I've seen this clue in The Independent.
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