Third letter in the Greek alphabet (5)
I believe the answer is:
gamma
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">third letter in the greek alphabet</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 gamma that I've seen before include "Letter from Greece" , "A bit of Greek" , "Greek letter - type of rays" , "Greek letter G" , "third highest grade" .)