That mineral of mine (3)
I believe the answer is:
ore
'that mineral' is the definition.
(ore is a kind of mineral)
'of mine' is the wordplay.
'of' becomes 'o''.
'mine' becomes 're' (I can't justify this - if you can you should believe this answer much more).
'o'+'re'='ORE'
Can you help me to learn more?
(Other definitions for ore that I've seen before include "not worth a lot to Danes" , "Deposit of mine" , "Raw iron" , "Mineral containing valuable metal" , "Mineral deposit" .)