Tailor set to become Plato's pupil (9)
I believe the answer is:
aristotle
'plato's pupil' is the definition.
(I've seen this in another clue)
'tailor set to become' is the wordplay.
'to become' indicates anagramming the letters.
'tailor'+'set'='tailorset'
'tailorset' is an anagram of 'ARISTOTLE'.
(Other definitions for aristotle that I've seen before include "Old Greek teacher" , "polymath" , "Totaliser (anag) - ancient Greek" , "Philosopher - treats oil (anag)" , "Ancient Athenian philosopher, pupil of Plato" .)