Lower luminosity initially in dark -- by this (9)
I believe the answer is:
moonlight
'this' is the definition.
The definition suggests a singular noun which matches the answer.
'lower luminosity initially in dark' is the wordplay.
'lower' becomes 'moo' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'initially' says to take the initial letters.
'in' indicates putting letters inside.
'dark' becomes 'night' (it's dark at nighttime).
The first letter of 'luminosity' is 'l'.
'l' put into 'night' is 'nlight'.
'moo'+'nlight'='MOONLIGHT'
'by' is the link.
Can you help me to learn more?
(Other definitions for moonlight that I've seen before include "work illegally" , "Beethoven composition for piano" , "do an extra job" , "Secretly work" , "name for sonata" .)