Plato's teacher (8)
I believe the answer is:
socrates
'plato's teacher' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for socrates that I've seen before include "Deep thinker" , "Coarsest one of great Greek philosophers" , "Gk. philosopher, took hemlock" , "Philosophical Greek" , "Coarsest sort of Greek philosopher" .)