Second-class silver in sack (3)
I believe the answer is:
bag
'sack' is the definition.
(I know this)
'second-class silver' is the wordplay.
'second-class' becomes 'b' ().
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'b'+'ag'='BAG'
'in' acts as a link.
(Other definitions for bag that I've seen before include "Secure" , "Claim" , "Carrier" , "Sack" , "The amount of game shot by sportsman" .)