Greek geometer (6)
I believe the answer is:
euclid
'greek geometer' 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 "mathematician appears" , "6 mathematician" , "Greek mathematician, 3rd century BC" , "Geometrician of the 3rd century BC" , "Greek mathematician, d. mid-3rd century BC" .)