Gaelic poetry lost first (4)

I believe the answer is:

erse

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'gaelic' is the definition.
(I know that gaelic can be written as Erse)

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for erse that I've seen before include "Irish Gaelic language" , "Old Celtic language" , "Possibly the language" , "Kind of Gaelic for seer" , "Old Scots or Irish Gaelic" .)

I've seen this clue in the Sydney Morning Herald.

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