P, to Pythagoras (3)
I believe the answer is:
rho
'p to pythagoras' 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 "Seventeenth letter of the Greek alphabet" , "17th Greek letter" , "letter to Athens" , "Gr. letter written as P" , "Sophocles character" .)