Greek mathematician (6)
'greek mathematician' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for euclid that I've seen before include "Greek mathematician c. three hundred BC" , "Great Greek mathematician, notable for geometry" , "6 mathematician" , "Early Greek geometrician" , "Ancient Greek master of geometry" .)
(Other definitions for pythagoras that I've seen before include "Ay to graphs (anag) - 21 3 17 theorist" , "Greek philosopher and mathematician, d. c 500 BC" , "Greek philosopher/mathematician" , "theorem by this guy" , "Greek with a theorem" .)