Small, silver sink (3)
I believe the answer is:
sag
'sink' is the definition.
(I know that sag is a more specific form of sink)
'small silver' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
's'+'ag'='SAG'
(Other definitions for sag that I've seen before include "Slump" , "Drop" , "Lose energy" , "Sinking" , "Spinach as used in Indian cookery" .)