Cuss (5)
I believe the answer is:
swear
'cuss' is the definition.
(I know that cuss can be written as swear)
This is the entire clue.
(Other definitions for swear that I've seen before include "Utter obscenities or take an oath" , "Eff and blind" , "Express annoyance" , "Use bad language" , "Use indecent or blasphemous language" .)