Small, dark and bright (6)
I believe the answer is:
sunlit
'bright' is the definition.
('sunlit' can be similar in meaning to 'bright')
'small dark' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'dark' becomes 'unlit' ('unlit' can be similar in meaning to 'dark').
's'+'unlit'='SUNLIT'
'and' is the link.
(Other definitions for sunlit that I've seen before include "enlightened by star" , "Illuminated by Sol" , "Unclouded" , "Insult (anag.)" , "Brightly illuminated" .)