Language in ruder sentence (4)
I believe the answer is:
erse
'language' is the definition.
(historical term for Scottish Gaelic)
'in ruder sentence' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ERSE' can be found hidden inside 'ruder sentence'.
(Other definitions for erse that I've seen before include "Possibly the language" , "Old name for Celtic languages" , "Old Celtic language" , "Old Gaelic for the seer" , "Celtic tongue" .)