It's illuminating for those in the dark (9)
I believe the answer is:
moonlight
'dark' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for moonlight that I've seen before include "Perform work at night" , "Lunar rays seen in thin gloom" , "Beethoven sonata" , "Beethoven's Op. twenty seven/two sonata" , "Night-time illumination" .)