models.py

from django.db import models

from jsonfield import JSONField


class ModelA(models.Model):
  data = models.IntegerField()
  extra = JSONField

serializers.py

from app.models import ModelA


class ExtraSerializer(serializers.Serializer):
  fieldA = serializers.CharField()
  fieldB = serializers.CharField()
  fieldC = serializers.CharField()
  
  def validate_fieldA(self, attrs, source):
    # Validate and raise ValidationError
    pass
  

class SerializerA(serializers.ModelSerializer):
  extra = ExtraSerializer(write_only=True)
  
  class Meta:
    model = ModelA
    
  def save_object(self, obj, **kwargs):
    # Hacky hacky!
    obj._nested_forward_relations.pop('extra')
    return super(SerializerA, self).save_object(obj, **kwargs)