Naturally bright or a bit slightly dark (6)
I believe the answer is:
sunlit
'naturally bright' is the definition.
(I've seen this in another clue)
'a bit slightly dark' is the wordplay.
'a bit slightly' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'dark' becomes 'unlit' (similar in meaning).
's'+'unlit'='SUNLIT'
'or' is the link.
Can you help me to learn more?
(Other definitions for sunlit that I've seen before include "Unclouded" , "Insult (anag.)" , "enlightened by star" , "In full daylight" , "Illuminated by daylight" .)