Start to generate blue radiance (4)
I believe the answer is:
glow
'radiance' is the definition.
(I know that radiance can be written as glow)
'start to generate blue' is the wordplay.
'start to' says to take the initial letters.
'blue' becomes 'low' (both can mean sad).
The first letter of 'generate' is 'g'.
'g'+'low'='GLOW'
(Other definitions for glow that I've seen before include "Redness of the cheeks" , "Shine steadily and brightly" , "warmth of feeling" , "Emanate light" , "Steady radiance" .)