Fantastic sort of light: legendary (6)
I believe the answer is:
fabled
'legendary' is the definition.
('fabled' can be a synonym of 'legendary')
'fantastic sort of light' is the wordplay.
'fantastic' becomes 'fab'.
'sort of light' becomes 'led' (light emitting diode).
'fab'+'led'='FABLED'
(Other definitions for fabled that I've seen before include "Enshrined in ancient stories or legends" , "Told of in legends or myths" , "Legendary, mythical" , "Mythical; imaginary" , "Well-known" .)