A barge is not as heavy (7)
I believe the answer is:
lighter
I believe this clue is a double definition.
'a barge' is the first definition.
('lighter' can be a synonym of 'barge')
'not as heavy' is the second definition.
('light' can be the opposite of 'heavy' so perhaps 'lighter' is reasonable for 'not as heavy')
'is' acts as a link.
(Other definitions for lighter that I've seen before include "Ship to shore boat" , "Boat with a flat bottom for carrying heavy loads" , "relieved of some cargo?" , "Flat-bottomed barge - not so heavy" , "Barge (more likely to float?)" .)