Greek geometer (6)
I believe the answer is:
euclid
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">greek geometer</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 euclid that I've seen before include "mathematician appears" , "6 mathematician" , "Greek mathematician, 3rd century BC" , "Geometrician of the 3rd century BC" , "Greek mathematician, d. mid-3rd century BC" .)