Prisoner left silver (3)
I believe the answer is:
lag
'prisoner' is the definition.
(slang term)
'left silver' is the wordplay.
'left' becomes 'l' (common abbreviation).
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'l'+'ag'='LAG'
(Other definitions for lag that I've seen before include "Fall behind on march" , "Dawdle" , "Fall behind; insulate" , "Prisoner" , "person inside" .)