Where each August competitive sailors may be offensive (8)
I believe the answer is:
insolent
'be offensive' is the definition.
(thesaurus)
I don't understand how the remainder of the clue works.
(Other definitions for insolent that I've seen before include "Cheeky" , "Impudent and disrespectful" , "Extremely fresh" , "Being discourteous and insulting" , "Pert" .)