Second-best element (6)
I believe the answer is:
silver
This 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 an element). I'm unsure of 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" .)