I believe the answer is:
'gaelic' is the definition.
(I know that gaelic can be written as Erse)
This is the entire clue.
(Other definitions for erse that I've seen before include "Name sometimes given to languages of the Celts in Ireland and Scotland" , "Old Celtic language" , "Old Scots or Irish Gaelic" , "Kind of Gaelic for seer" , "Possibly the language" .)