One should receive luminance in the dark (5)
I believe the answer is:
unlit
'in the dark' is the definition.
(I've seen this before)
'one should receive luminance' is the wordplay.
'one' becomes 'unit' ('unit' can be a synonym of 'one').
'should receive' means one lot of letters goes inside another.
'luminance' becomes 'l' (physics abbreviation).
'unit' enclosing 'l' is 'UNLIT'.
(Other definitions for unlit that I've seen before include "Not yet fired up" , "With no lighting" , "such as dark alley?" , "Left in the dark" , "Without illumination" .)