Sixth Greek letter (4)
I believe the answer is:
zeta
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">sixth greek letter</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for zeta that I've seen before include "Sixth letter of the Greek alphabet" , "Spartan character" , "Welsh actress, Catherine __-Jones, b. 1969" , "Greek character" , "Greek letter (for Catherine Jones?)" .)