Very old silver, the last 500 (4)
I believe the answer is:
aged
'very old' is the definition.
(I've seen this before)
'silver the last 500' is the wordplay.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'last' suggests the final letters.
'500' becomes 'D' (Roman numeral for 500).
The final letter of 'the' is 'e'.
'ag'+'e'+'d'='AGED'
(Other definitions for aged that I've seen before include "Antiquated" , "Very old, person or wine" , "Time-worn" , "not like a callow youth" , "Long in the tooth" .)