Realist to become a Greek philosopher (9)
I believe the answer is:
aristotle
'a greek philosopher' is the definition.
(I've seen this before)
'realist to become' is the wordplay.
'become' indicates an anagram.
'realist'+'to'='realistto'
'realistto' anagrammed gives 'ARISTOTLE'.
(Other definitions for aristotle that I've seen before include "noted thinker" , "Totaliser (anag) - ancient Greek" , "Philosopher - treats oil (anag)" , "philosophy tutor" , "Ancient Athenian philosopher, pupil of Plato" .)