Language upsets seer (4)
I believe the answer is:
erse
'language' is the definition.
(historical term for Scottish Gaelic)
'upsets seer' is the wordplay.
'upsets' is an anagram indicator.
'seer' with letters rearranged gives 'ERSE'.
(Other definitions for erse that I've seen before include "Old Celtic language" , "Possibly the language" , "Old Scots or Irish Gaelic" , "Old Gaelic for the seer" , "Kind of Gaelic for seer" .)