Copying brooch in silver (5)
I believe the answer is:
aping
'copying' is the definition.
('ape' can be a synonym of 'copy')
'brooch in silver' is the wordplay.
'brooch' becomes 'pin' (decorative item pinned to clothing).
'in' is an insertion indicator.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'pin' placed inside 'ag' is 'APING'.
(Other definitions for aping that I've seen before include "Taking off" , "Acting like a parrot" , "Sending up" , "Parroting" , "Making copy of" .)