Gael's tongue (4)
I believe the answer is:
erse
'gael's tongue' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for erse that I've seen before include "Possibly the language" , "Old language spoken by strange seer" , "Old name for Celtic languages" , "Irish Gaelic language" , "Celtic tongue" .)