Start generating below-average radiance (4)
I believe the answer is:
glow
'radiance' is the definition.
(I know that radiance can be written as glow)
'start generating below-average' is the wordplay.
'start' suggests taking the first letters.
'below-average' becomes 'low' (I can't explain this - if you can you should give a lot more credence to this answer).
The initial letter of 'generating' is 'g'.
'g'+'low'='GLOW'
Can you help me to learn more?
(Other definitions for glow that I've seen before include "Shine with heat" , "Shine with warmth, emotion" , "Give out light without flame" , "Radiate or redden" , "Give out steady light" .)