Between rho and tau (5)
I believe the answer is:
sigma
'between rho and tau' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for sigma that I've seen before include "Agism of the Greek letter" , "Letter from Athens" , "character in Corfu" , "Greek letter indicating summation in maths" , "local opening in Salonika" .)