Left silver for prisoner (3)
I believe the answer is:
lag
'prisoner' is the definition.
(I've seen this before)
'left silver' is the wordplay.
'left' becomes 'l' (common abbreviation).
'silver' becomes 'ag' ('Ag' can be a synonym of 'silver').
'l'+'ag'='LAG'
'for' acts as a link.
(Other definitions for lag that I've seen before include "Fall behind; insulate" , "Fall behind or cover to prevent heat loss" , "Convict (slang)" , "Fall behind with gal" , "Loiter" .)