Greek letter S (5)
I believe the answer is:
sigma
'greek letter s' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sigma that I've seen before include "Greek letter corresponding to S" , "Athenian character" , "Greek letter signifying sum in maths" , "How Socrates ended" , "Greek letter indicating summation in maths" .)