Rude — too rude — I cuss terribly (12)
I believe the answer is:
discourteous
'rude' is the definition.
(I've seen this before)
'too rude i cuss terribly' is the wordplay.
'terribly' indicates an anagram.
'too'+'rude'+'i'+'cuss'='toorudeicuss'
'toorudeicuss' is an anagram of 'DISCOURTEOUS'.
(Other definitions for discourteous that I've seen before include "Ill-mannered" , "Impolite" , "Rude, inconsiderate" , "Uncouth" .)