Not as heavy (7)
I believe the answer is:
lighter
'not as heavy' is the definition.
('light' can be the opposite of 'heavy' so perhaps 'lighter' is reasonable for 'not as heavy')
This is all the clue.
(Other definitions for lighter that I've seen before include "Vessel" , "Smoking accessory" , "Open boat - spill" , "The girl (anag.)" , "Cargo barge; smoker's accessory" .)