Gaelic language (4)
I believe the answer is:
'gaelic language' is the definition.
(term for the Scottish or Irish language)
This is the entire clue.
(Other definitions for erse that I've seen before include "Old Scots or Irish Gaelic" , "Old name for Celtic languages" , "Possibly the language" , "Name sometimes given to languages of the Celts in Ireland and Scotland" , "Old language spoken by strange seer" .)