Not so heavy smoker's accessory (7)
I believe the answer is:
lighter
I believe this clue is a double definition.
'not so heavy' is the first definition.
('light' can be the opposite of 'heavy' so perhaps 'lighter' is reasonable for 'not so heavy')
'smoker's accessory' is the second definition.
(a cigarette lighter)
(Other definitions for lighter that I've seen before include "that leads the way for others?" , "more blonde" , "Barge - less heavy" , "Supply boat" , "Barge - alternative to match" .)