Light-emitting diode was in front (3)
I believe the answer is:
I believe this is a double definition.
'light-emitting diode' is the first definition.
'was in front' is the second definition.
(I've seen this in another clue)
(Other definitions for led that I've seen before include "Took by the hand and showed the way" , "Went at the head of others" , "Emitter of light" , "Went in advance of others" , "had the edge" .)