Greek mathematician, the 'father of geometry' (6)
I believe the answer is:
euclid
I believe this clue is a double definition.
'greek mathematician' is the first definition.
(I've seen this before)
'the father of geometry' is the second definition.
(I've seen this in another clue)
(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" .)