We're told progeny is a source of light (3)
I believe the answer is:
sun
'source of light' is the definition.
(I've seen this in another clue)
'we're told progeny' is the wordplay.
'we're told' shows a homophone (sound like).
'progeny' becomes 'son' (son is a kind of progeny).
'son' is a homophone of 'SUN'.
'is a' acts as a link.
(Other definitions for sun that I've seen before include "Dawn riser" , "Helios, in Greek myth" , "Star; paper" , "Earth's nearest star" , "Star round which we orbit" .)