New silver horse (3)
I believe the answer is:
nag
'horse' is the definition.
(term for an old horse)
'new silver' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'n'+'ag'='NAG'
(Other definitions for nag that I've seen before include "trouble constantly" , "shrew or > badger" , "Worry constantly" , "Persistently worry; horse" , "Ailing horse" .)