Greek astronomer (7)
I believe the answer is:
'greek astronomer' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ptolemy that I've seen before include "Greek astronomer; Greco-Egyptian ruler" , "Egyptian king or astronomer" , "Egyptian ruler; Greek astronomer" , "Ancient astronomer" , "Alexandrian astronomer" .)