Second-best element (6)
I believe the answer is:
silver
This clue might 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 a kind of element). I'm 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" .)