Radiant source: that's our paper! (3)
I believe the answer is:
sun
'paper' is the definition.
(The Sun, UK newspaper)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for sun that I've seen before include "Centre of Earth's orbit" , "Source of light and heat" , "Bask" , "Earth's nearest star" , "Warmer" .)