Letter before tau (5)
I believe the answer is:
sigma
'letter before tau' 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 "Greek letter indicating summation in maths" , "Greek letter used as a symbol for summation" , "Greek letter corresponding to S" , "character in Athens" , "Foreign character" .)