Copper turned green as mint, in the end (8)
I believe the answer is:
sergeant
'copper' is the definition.
(I've seen this before)
'turned green as mint in the end' is the wordplay.
'turned' is an anagram indicator.
'in the end' indicates one should take the final letters.
The final letter of 'mint' is 't'.
'green'+'as'='greenas'
'greenas' with letters rearranged gives 'sergean'.
'sergean'+'t'='SERGEANT'
(Other definitions for sergeant that I've seen before include "Troy" , "Estrange (anag)" , "Reagents for army NCO" , "Estrange from junior army officer" , "Noncommissioned military officer" .)