Language of poetry beginning to wither away (4)

I believe the answer is:

erse

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'language' is the definition.
(historical term for Scottish Gaelic)

I don't understand the remainder of the clue.

(Other definitions for erse that I've seen before include "Name sometimes given to languages of the Celts in Ireland and Scotland" , "Old language spoken by strange seer" , "Kind of Gaelic for seer" , "Old Scots or Irish Gaelic" , "Old name for Celtic languages" .)

I've seen this clue in The Guardian.

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