Sigma preceder (3)
I believe the answer is:
rho
'sigma preceder' 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 "letter to Athens" , "Greek P" , "Sophocles character" , "Gr. letter written as P" , "Greek letter used as symbol for density" .)