Letter before sigma (3)
I believe the answer is:
rho
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">letter before sigma</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for rho that I've seen before include "Greek letter used as symbol for density" , "Greek P" , "Something like 25 [ETA]" , "Seventeenth letter of the Greek alphabet" , "Greek letter R" .)