Good base for light (4)
I believe the answer is:
glow
'for light' is the definition.
(glow is a kind of light)
'good base' is the wordplay.
'good' becomes 'g' (abbreviation).
'base' becomes 'low' (synonyms).
'g'+'low'='GLOW'
(Other definitions for glow that I've seen before include "Steady radiance" , "Incandescence" , "Give out steady light" , "Gentle, warm light" , "Shine with warmth, emotion" .)