Greek poem, one backing Irish parliament (5)
I believe the answer is:
iliad
'greek poem' is the definition.
(I've seen this before)
'one backing irish parliament' is the wordplay.
'one' becomes 'i' (Roman numeral).
'backing' says the letters should be written in reverse.
'irish parliament' becomes 'dail' (I've seen this before).
'dail' backwards is 'liad'.
'i'+'liad'='ILIAD'
(Other definitions for iliad that I've seen before include "Epic poem attributed to Homer" , "Epic piece of writing" , "set of books" , "Greek epic poem about the Trojan War" , "Great poem by Homer" .)