Letter before sigma (3)
I believe the answer is:
rho
'letter before sigma' 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 "Greek letter used as symbol for density" , "Greek P" , "Something like 25 [ETA]" , "Seventeenth letter of the Greek alphabet" , "Greek letter R" .)