Came first, being sort of light (3)
I believe the answer is:
led
I believe this clue is a double definition.
'came first' is the first definition.
(I've seen this before)
'being sort of light' is the second definition.
(light emitting diode)
(Other definitions for led that I've seen before include "Shown the way" , "Conducted" , "Came first" , "Went at the head of others" , "showed the way ahead" .)