Less serious (7)
I believe the answer is:
lighter
'less serious' is the definition.
('light' can be the opposite of 'serious' so perhaps 'lighter' is reasonable for 'less serious')
This is the entire clue.
(Other definitions for lighter that I've seen before include "Device for igniting cigarette" , "Kind of barge - carrying less?" , "Barge - alternative to match" , "Goods barge" , "craft" .)