Mathematician's summation symbol (5)
I believe the answer is:
sigma
'mathematician's summation symbol' 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 "Greek character" , "Greek letter corresponding to S" , "Letter from Greece" , "Greek letter used as a symbol for summation" , "Agism of the Greek letter" .)