Mineral found in no other mine ultimately (3)
I believe the answer is:
ore
'mineral' is the definition.
(ore is a kind of mineral)
'no other mine ultimately' is the wordplay.
'ultimately' indicates one should take the final letters.
The last letters of 'no other mine' are 'ORE'.
'found in' is the link.
(Other definitions for ore that I've seen before include "14 [CURRENCY UNIT]" , "Small coins" , "tangle" , "Danish currency" , "Mineral-bearing material" .)