Ignition device not as heavy (7)
I believe the answer is:
lighter
This clue is a double definition.
'ignition device' is the first definition.
(device for igniting or lighting something)
'not as heavy' is the second definition.
('light' can be the opposite of 'heavy' so perhaps 'lighter' is reasonable for 'not as heavy')
(Other definitions for lighter that I've seen before include "Cargo barge; smoker's accessory" , "Canal boat with a flat bottom" , "- boot; - bun; - ware" , "more blonde" , "Ship to shore boat" .)