One youngster taking in one classical poem (5)
I believe the answer is:
iliad
'classical poem' is the definition.
(I know that Iliad is Graeco-Roman)
'one youngster taking in one' is the wordplay.
'one' becomes 'I' (Roman numeral).
'youngster' becomes 'lad' (a young boy).
'taking in' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'lad' enclosing 'i' is 'liad'.
'i'+'liad'='ILIAD'
(Other definitions for iliad that I've seen before include "ancient tale" , "Homer's tale" , "The -----, Homeric epic" , "story of Mediterranean heroism" , "set of books" .)