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