The —, Greek epic poem describing the siege of Troy (5)
I believe the answer is:
iliad
'the greek epic poem describing the siege of troy' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for iliad that I've seen before include "Homer's tale" , "Homer's great poem" , "A work by 7" , "long story" , "Epic piece of writing" .)