It's bright - when son leaves, it becomes dark (6)
I believe the answer is:
sunlit
'it's bright' is the definition.
(similar in meaning)
'becomes dark' is the wordplay.
'becomes' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'dark' becomes 'unlit' (similar in meaning).
's'+'unlit'='SUNLIT'
'when son leaves it' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for sunlit that I've seen before include "In full daylight" , "Not so at night" , "Hit by solar rays" , "Unclouded" , "facilitating one's vision, naturally" .)