Greek letter S (5)
I believe the answer is:
sigma
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">greek letter s</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)