Not as heavy as a barge (7)
I believe the answer is:
lighter
I believe this is a double definition.
'not as heavy' is the first definition.
('light' can be the opposite of 'heavy' so perhaps 'lighter' is reasonable for 'not as heavy')
'a barge' is the second definition.
('lighter' can be a synonym of 'barge')
'as' acts as a link.
(Other definitions for lighter that I've seen before include "that leads the way for others?" , "Kind of barge - carrying less?" , "Fire-raiser" , "carrying less cargo?" , "less tonnage" .)