Epic poem is long. It's also dull, initially (5)
I believe the answer is:
iliad
'epic poem' is the definition.
(Iliad is an example)
'is long it's also dull initially' is the wordplay.
'initially' suggests taking the first letters.
The first letters of 'is long its also dull' is 'ILIAD'.
(Other definitions for iliad that I've seen before include "epic work" , "Homer's work" , "Homer Trojan war epic" , "ancient tale" , "A work by 7" .)