Virgil's first language: poetry (5)
I believe the answer is:
verse
'poetry' is the definition.
(I know that poetry can be written as verse)
'virgil's first language' is the wordplay.
'virgil's first' becomes 'v' (est letter of 'virgil').
'language' becomes 'erse' (historical term for Scottish Gaelic).
'v'+'erse'='VERSE'
(Other definitions for verse that I've seen before include "Metrical text" , "few lines of writing?" , "Poetry - subdivision of the Bible" , "Section of a poem" , "Poems" .)