One young man penning one long poem (5)
I believe the answer is:
iliad
'long poem' is the definition.
(I've seen this in another clue)
'one young man penning one' is the wordplay.
'one' becomes 'i' (Roman numeral).
'young man' becomes 'lad' (lad can mean a boy or young man).
'penning' 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 epic poem on the siege of Troy" , "ancient tale" , "Greek epic poem describing the siege of Troy" , "Homer's ancient Greek epic" , "Poem by 4 across" .)