I think you want to override the clean() method on your model rather than the clean method of a specific form field. From the docs -
This method should be used to provide custom model validation, and to modify attributes on your model if desired. For instance, you could use it to automatically provide a value for a field, or to do validation that requires access to more than a single field.
If you did want to put the validation in the form then the clean()
method on the form provides similar functionality (see docs).