Large open boat is not so dark (7)
I believe the answer is:
lighter
I believe this is a double definition.
'large open boat' is the first definition.
'not so dark' is the second definition.
('light' can be the opposite of 'dark' so perhaps 'lighter' is reasonable for 'not so dark')
'is' is the link.
(Other definitions for lighter that I've seen before include "Cargo barge; smoker's accessory" , "Barge - alternative to match" , "Barge - less heavy" , "Kind of barge - carrying less?" , "Barge; one needed by smoker" .)