Django rest how clear any specific fields after save serializer data

I want to clear my captcha fields after save my serializer data.

    contact_name = models.CharField(max_length=100)
    contact_email = models.EmailField(max_length=200)
    contact_body = models.TextField()
    captcha = models.CharField(max_length=2000, blank=True, null=True)

class ContactSerializer(serializers.ModelSerializer):
    class Meta:
        model = Contact
        fields = '__all__'

@api_view(['POST', 'GET'])
def contact_api(request):
    if request.method == 'POST':
        captcha ="captcha")
        data =

        serializer = ContactSerializer(data=data)

        if serializer.is_valid():
            ''' Begin reCAPTCHA validation '''
           #my server side captcha verification code...
            if result['success']:
                # here I want to clear my captcha fields 
                return Response({
                    'status': True,
                    'message': "sucess"

            return Response(status=status.HTTP_404_NOT_FOUND)

Why do you want to clear the field?

I want to clear this for an specific reason. is it possible? if yes then how?

You can modify the instance behind the serialized via serializer.instance.