Label bit of tarnished silver (3)
I believe the answer is:
tag
'label' is the definition.
(I know that label can be written as tag)
'bit of tarnished silver' is the wordplay.
'bit of' says to take the initial letters.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
The initial letter of 'tarnished' is 't'.
't'+'ag'='TAG'
(Other definitions for tag that I've seen before include "Identity disc" , "Label (to attach)" , "Tie-on label" , "Small label" , "dog" .)