Darkness arising when there's no good light (4)
I believe the answer is:
thin
'light' is the definition.
(similar in meaning)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for thin that I've seen before include "Insubstantial" , "Watered down" , "Measure" , "Lean; make (plants) less crowded" , "Slim - watery" .)