Not as serious as an incendiary device (7)
I believe the answer is:
lighter
This clue might be a double definition.
'not as serious' is the first definition.
('light' can be the opposite of 'serious' so perhaps 'lighter' is reasonable for 'not as serious')
'incendiary device' is the second definition.
'lighter' can be an answer for 'device' (lighter is a kind of device). I'm not sure about the 'incendiary' bit.
'as an' is the link.
Can you help me to learn more?
(Other definitions for lighter that I've seen before include "Vessel" , "Boat for heavy loads and fire igniter" , "carrying less cargo?" , "Cargo barge; smoker's accessory" , "Less weighty" .)