One lad pens one war story (5)
I believe the answer is:
iliad
'war story' is the definition.
(I've seen this in another clue)
'one lad pens one' is the wordplay.
'one' becomes 'i' (Roman numeral).
'pens' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'lad' enclosing 'i' is 'liad'.
'i'+'liad'='ILIAD'
(Other definitions for iliad that I've seen before include "Homer's ancient Greek epic" , "LITERATURE" , "A work by 7" , "Old story" , "City investment story" .)