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