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