Less weighty (7)
I believe the answer is:
lighter
'less weighty' is the definition.
('light' can be the opposite of 'weighty' so perhaps 'lighter' is reasonable for 'less weighty')
This is all the clue.
(Other definitions for lighter that I've seen before include "Barge; igniter" , "- boot; - bun; - ware" , "Boat with a flat bottom for carrying heavy loads" , "Supply boat" , "carrying less weight" .)