Five Gaelic lines of poetry (5)
I believe the answer is:
verse
'lines of poetry' is the definition.
'five gaelic' is the wordplay.
'five' becomes 'V' (Roman numeral).
'gaelic' becomes 'Erse' (synonyms).
'v'+'erse'='VERSE'
(Other definitions for verse that I've seen before include "Part of a song or poem" , "Metrical text" , "Literature in metrical form" , "few lines of writing?" , "Poetry - subdivision of the Bible" .)