Greek letter O (7)
I believe the answer is:
omicron
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">greek letter o</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for omicron that I've seen before include "Greek vowel" , "Round character" , "A rounded character" , "letter from Athens" , "character in Sophocles" .)