Plato's "P" (3)
I believe the answer is:
rho
'plato's p' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rho that I've seen before include "Sophocles character" , "Seventeenth letter of the Greek alphabet" , "Greek character" , "Greek letter R" , "Something like 25 [ETA]" .)