Metal said to have been a conductor (3)

I believe the answer is:

led

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'conductor' is the definition.
(I know that LED is a type of crystal rectifier)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for led that I've seen before include "Went in advance of others" , "Something emitting light" , "Went first to show the way" , "Paved the way" , "Took by the hand and showed the way" .)

I've seen this clue in The Globe and Mail.

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