Ancient poet to observe grass in Irish county, piled up (4,7)
I believe the answer is:
omar khayyam
'ancient poet' is the definition.
'omar khayyam' can be an answer for 'poet' (I have seen 'Persian poet' mean 'omar khayyam' so perhaps 'poet' could also mean 'omar khayyam'). I'm unsure of the 'ancient' bit.
'observe grass in irish county piled up' is the wordplay.
'observe' becomes 'mark' (marking is a kind of observing).
'grass' becomes 'hay' (hay is dried grass).
'in' is an insertion indicator.
'irish county' becomes 'mayo' (county in Ireland).
'piled up' says the letters should be written in reverse.
'mayo' in reverse letter order is 'oyam'.
'mark'+'hay'='markhay'
'markhay' put inside 'oyam' is 'OMAR KHAYYAM'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for omar khayyam that I've seen before include "Persian poet" , "Poet translated by Edward Fitzgerald" , "Persian rubaiyat writer" .)