A Welsh girl possibly penning English epic (6)
I believe the answer is:
aeneid
'epic' is the definition.
(epic poem by Virgil)
'a welsh girl possibly penning english' is the wordplay.
'welsh girl possibly' becomes 'enid' (common Welsh name).
'penning' indicates putting letters inside.
'english' becomes 'e' (abbreviation).
'enid' placed around 'e' is 'eneid'.
'a'+'eneid'='AENEID'
(Other definitions for aeneid that I've seen before include "Virgil's epic Latin poem" , "Latin poetry" , "Virgilian epic poem" , "epic from the past" , "Work" .)