Not so heavy (7)
I believe the answer is:
lighter
'not so heavy' is the definition.
('light' can be the opposite of 'heavy' so perhaps 'lighter' is reasonable for 'not so heavy')
This is all the clue.
(Other definitions for lighter that I've seen before include "Goods barge" , "carrying less weight" , "Barge; flame producer" , "Less profound" , "smoker may resort to it" .)