Plato's mentor (8)
I believe the answer is:
socrates
'plato's mentor' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for socrates that I've seen before include "old question-master?" , "Coarsest sort of Greek philosopher" , "Man philosophically questioning" , "Deep thinker" , "Hemlocked philosopher" .)