Brusque ambassador abandoning the Irish language (5)
I believe the answer is:
terse
'brusque' is the definition.
(thesaurus)
'ambassador abandoning the irish language' is the wordplay.
'ambassador' becomes 'he' (His or Her Excellency).
'abandoning' indicates named letters should be taken away.
'irish language' becomes 'erse' (I've seen this before).
'the' with 'he' taken out is 't'.
't'+'erse'='TERSE'
(Other definitions for terse that I've seen before include "Succinct about trees" , "Pithy and concise" , "Reset to be pithy" , "Brief; abrupt" , "Brusque among trees" .)