Low luminance in the dark – bright luminance in the dark? (9)
I believe the answer is:
moonlight
'bright luminance in the dark?' is the definition.
(light at night time)
'low luminance in the dark' is the wordplay.
'low' becomes 'moo' (both can mean to make the noise of a cow).
'luminance' becomes 'L' (physics abbreviation).
'in' indicates putting letters inside.
'the dark' becomes 'night' (it's dark at nighttime).
'l' going into 'night' is 'nlight'.
'moo'+'nlight'='MOONLIGHT'
(Other definitions for moonlight that I've seen before include "Lunar rays seen in thin gloom" , "Do extra work" , "Work late" , "Secretly work" , "Beethoven sonata" .)