Match not serious (5)
I believe the answer is:
light
I believe this clue is a double definition.
'match' is the first definition.
(I've seen this before)
'not serious' is the second definition.
('light' is the opposite to 'serious')
(Other definitions for light that I've seen before include "Weightless; lamp" , "Airy; not heavy" , "Form of radiation - not serious" , "weighing relatively little" , "Undemanding; crossword answer" .)