Match, for example, that's not serious (5)
I believe the answer is:
light
I believe this is a double definition.
'match for example' is the first definition.
(match is a kind of light)
'not serious' is the second definition.
('light' is the opposite to 'serious')
'that's' is the link.
(Other definitions for light that I've seen before include "Not having full load" , "Window; insubstantial" , "solution to clue" , "Falling short of contract" , "Illuminate or ignite" .)