P, for Plato (3)
I believe the answer is:
rho
'p for plato' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rho that I've seen before include "Seventeenth letter of the Greek alphabet" , "Gr. letter written as P" , "17th Greek letter" , "letter to Athens" , "Greek letter used as symbol for density" .)