Old silver found by Miliband (4)
I believe the answer is:
aged
'old' is the definition.
(similar in meaning)
'silver found by miliband' is the wordplay.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'found by' is a charade indicator (letters next to each other).
'miliband' becomes 'ed' (politician Ed Miliband).
'ag'+'ed'='AGED'
(Other definitions for aged that I've seen before include "well over the hill" , "Matured like wine in wood" , "Elderly" , "not like a callow youth" , "Geriatric" .)