Rho follower (5)
I believe the answer is:
sigma
'rho follower' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sigma that I've seen before include "Letter from Greece" , "Greek letter indicating summation in maths" , "Greek letter used as a symbol for summation" , "Athenian character" , "Gr. letter; rhymes with mark of disgrace" .)