Use foul language
(Other definitions for curse that I've seen before include "Imprecate" , "Wish evil upon" , "Excommunication" , "Oath or a call for evil to come upon person" , "Invocation of evil; swear" .)
'use foul language' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for cuss that I've seen before include "Blaspheme" , "Stubborn person" , "Use bad language (informal)" , "Swear (slang)" , "Oath; (annoying sort of) person" .)