Bright, but less so when son leaves (6)
I believe the answer is:
sunlit
'bright' is the definition.
(similar in meaning)
'less so when son leaves' is the wordplay.
I cannot really understand how this works, but
'son' could be 's' (genealogical abbreviation for son) and 's' is found in the answer.
The remaining letters 'unlit' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'but' acts as a link.
Can you help me to learn more?
(Other definitions for sunlit that I've seen before include "Bathed in natural light" , "Illuminated by daylight" , "Bright with solar illumination" , "Insult (anag.)" , "Provided with illumination, naturally" .)