Paper not serious, but it's illuminating (8)
I believe the answer is:
sunlight
'it's illuminating' is the definition.
'paper not serious' is the wordplay.
'paper' becomes 'sun' (The Sun, UK newspaper).
'not serious' becomes 'light' ('light' is the opposite to 'serious').
'sun'+'light'='SUNLIGHT'
'but' is the link.
(Other definitions for sunlight that I've seen before include "Prerequisite for photosynthesis" , "Solar radiation" , "What's illuminating, naturally" , "Solar rays" , "Beams" .)