Something highly desirable containing a metal or another (8)
I believe the answer is:
platinum
'something highly desirable' is the definition.
I can't tell whether this definition defines the answer.
'containing a metal or another' is the wordplay.
'containing' means one lot of letters goes inside another.
'metal' becomes 'tin' (tin is a kind of metal).
'or another' becomes 'plum' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'tin'='atin'
'atin' inserted within 'plum' is 'PLATINUM'.
Can you help me to learn more?
(Other definitions for platinum that I've seen before include "Valuable silvery metal" , "Metal used in jewellery" , "Heavy precious metallic element" , "Atomic number 78" , "Valuable stuff, after a grind" .)