Less weighty barge (7)
I believe the answer is:
lighter
I believe this clue is a double definition.
'less weighty' is the first definition.
('light' can be the opposite of 'weighty' so perhaps 'lighter' is reasonable for 'less weighty')
'barge' is the second definition.
(synonyms)
(Other definitions for lighter that I've seen before include "Fire-raiser" , "Smoker's implement" , "less tonnage" , "Barge; flame producer" , "Supply boat" .)