Aristotle's teacher (5)
I believe the answer is:
plato
'aristotle's teacher' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for plato that I've seen before include "Greek philosopher, disciple of Aristotle" , "Ancient Greek philosopher" , "Greek philosopher (The Republic)" , "To pal up with Greek philosopher" , "Republican writer?" .)