Precious metal for Long John (6)
I believe the answer is:
silver
I believe this is a double definition.
'precious metal' is the first definition.
(silver is a kind of precious metal)
'long john' is the second definition.
(I've seen this in another clue)
'for' acts as a link.
(Other definitions for silver that I've seen before include "treasure" , "Mineral ore, chemical symbol Ag" , "old folk?" , "Silver of precious metal" , "Argent" .)