Aristotle's teacher (5)
I believe the answer is:
plato
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">aristotle's teacher</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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?" .)