Second-best element (6)

I believe the answer is:

silver

I'm a little stuck... Click here to teach me more about this clue!

This could be a double definition.

'second' is the first definition.
(I've seen this before)

'best element' is the second definition.
'silver' can be an answer for 'element' (silver is an element). I am not sure about the 'best' bit.

Can you help me to learn more?

(Other definitions for silver that I've seen before include "Grey-haired" , "See 1" , "treasure" , "Precious metal (for second place)" , "Pirate" .)

I've seen this clue in The Mirror.

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