forms.py

class MyForm(forms.ModelForm):
    ...
    def __init__(self, *args, **kwargs):
        self.competition = kwargs.pop('competition')
        super(MyForm, self).__init__(*args, **kwargs)

    def clean(self):
        """ Use self.competition here """

views.py

...
competition = get_object_or_404(Competition, slug=competition_slug)
form = MyForm(request.POST, competition=competition)
...