Epic poem before AD 151? (5)

I believe the answer is:

iliad

I'm a little stuck... Click here to teach me more about this clue!

'epic poem' is the definition.
(Iliad is an example)

'before ad 151?' is the wordplay.
I cannot really understand how this works, but
'ad' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for iliad that I've seen before include "set of books" , "Classic" , "tale of woes" , "Homer's epic poem on the siege of Troy" , "ancient tale" .)

I've seen this clue in The Guardian.

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