A mined extract of platinum (3)
I believe the answer is:
tin
'a mined extract' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'of platinum' is the wordplay.
'of' indicates a hidden word.
'TIN' is hidden in the letters of 'platinum'.
Can you help me to learn more?
(Other definitions for tin that I've seen before include "A silvery-white metal often in cans" , "Silvery white metal" , "Light metal" , "Kind of metal often used as food container" , "roofing material" .)