Third Greek letter (5)
I believe the answer is:
gamma
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">third greek letter</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 gamma that I've seen before include "Letter after alpha, beta" , "A bit of Greek" , "Greek character" , "Poor mark" , "Foreign character" .)