Greek mathematician, the 'father of geometry' (6)
I believe the answer is:
euclid
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">greek mathematician</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">the father of geometry</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(Other definitions for euclid that I've seen before include "Alexandrian mathematician" , "Book of geometry" , "Early Greek geometrician" , "mathematician appears" , "Greek mathematician, d. mid-3rd century BC" .)