A canister placed in choice metal (8)
I believe the answer is:
platinum
'metal' is the definition.
(I know that platinum is a metal)
'a canister placed in choice' is the wordplay.
'canister' becomes 'tin' (synonyms).
'placed in' means one lot of letters goes inside another (I've seen this in other clues).
'choice' becomes 'plum' (both can mean excellent).
'a'+'tin'='atin'
'atin' inserted inside 'plum' is 'PLATINUM'.
(Other definitions for platinum that I've seen before include "Heavy precious metallic element" , "Jewellers' metal" , "Silver-white precious metal" , "Metal used in jewellery" , "-- Blonde; -- disc" .)