Letter after rho (5)
I believe the answer is:
sigma
'letter after rho' 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 "character in Corfu" , "character in Athens" , "Letter from Athens" , "Greek letter used as a symbol for summation" , "Greek letter indicating summation in maths" .)