Less serious match? (7)
I believe the answer is:
lighter
This clue is a double definition.
'less serious' is the first definition.
('light' can be the opposite of 'serious' so perhaps 'lighter' is reasonable for 'less serious')
'match?' is the second definition.
(match is a kind of lighter)
(Other definitions for lighter that I've seen before include "Flat-bottomed barge - not so heavy" , "Barge; flame producer" , "Cargo barge; smoker's accessory" , "Goods barge" , "Barge (more likely to float?)" .)