Ring again for a mineral (3)
I believe the answer is:
ore
'for a mineral' is the definition.
(ore is a kind of mineral)
'ring again' is the wordplay.
'ring' becomes 'O' (letter resembling a ring).
'again' becomes 're' (prefix meaning 'again').
'o'+'re'='ORE'
(Other definitions for ore that I've seen before include "It's mined for minerals" , "Raw metal" , "Raw material" , "Mineral deposit" , "store of value" .)