Luminous ultimately, except when dark (7)
I believe the answer is:
sunless
'dark' is the definition.
(I've seen this in another clue)
'luminous ultimately except' is the wordplay.
'ultimately' indicates one should take the final letters.
'except' becomes 'unless' (I've seen this in another clue).
The final letter of 'luminous' is 's'.
's'+'unless'='SUNLESS'
'when' is the link.
(Other definitions for sunless that I've seen before include "Offering nothing for a tanner" , "gloomy" , "Cloudy" , "dim, certainly not bright" , "Clouded over" .)