Rust-free extract of platinum (3)
I believe the answer is:
tin
'rust-free extract' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'of platinum' is the wordplay.
'of' indicates the answer is hidden within the clue.
'TIN' can be found hidden inside 'platinum'.
Can you help me to learn more?
(Other definitions for tin that I've seen before include "Kind of metal often used as food container" , "A silvery-white metal often in cans" , "Silvery-white metallic element" , "Metal in Argentina" , "loaf of bread" .)