The letter after pi (3)
I believe the answer is:
rho
'the letter after pi' 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" , "letter to Athens" , "Something like 25 [ETA]" , "Sophocles character" , "Greek letter used as symbol for density" .)