Poetry in very old language (5)
I believe the answer is:
verse
'poetry' is the definition.
(I know that poetry can be written as verse)
'very old language' is the wordplay.
'very' becomes 'v' (common abbreviation).
'old language' becomes 'erse' (old-fashioned term for Gaelic).
'v'+'erse'='VERSE'
'in' acts as a link.
(Other definitions for verse that I've seen before include "few lines of writing?" , "Part of a song or poem" , "literary offering" , "Piece of poetry" , "Poems" .)