Mastermind converted in Greene (8)
I believe the answer is:
engineer
'mastermind' is the definition.
(synonyms)
'converted in greene' is the wordplay.
'converted' is an anagram indicator.
'in'+'greene'='ingreene'
'ingreene' with letters rearranged gives 'ENGINEER'.
(Other definitions for engineer that I've seen before include "plan" , "Person in charge of building bridges, etc." , "One who designs, makes or runs motors" , "Mastermind" , "One uses scientific knowledge to solve practical problems" .)