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