Abrupt end to Celt language (5)
I believe the answer is:
terse
'abrupt' is the definition.
(I've seen this before)
'end to celt language' is the wordplay.
'end to' says to take the final letters.
'language' becomes 'erse' (historical term for Scottish Gaelic).
The last letter of 'celt' is 't'.
't'+'erse'='TERSE'
(Other definitions for terse that I've seen before include "Crisp and brief" , "Using few words" , "Short, abrupt" , "Crisply brief" , "Reset to be pithy" .)