Persian polymath Khayyám (4)
I believe the answer is:
omar
'persian polymath khayyam' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for omar that I've seen before include "and 16 Down: Persian poet" , "- Khayyam, Persian poet, astronomer and mathematician" , "old poet" , "- Sharif, film actor" , "Arab" .)