make django model field read only or disable in admin while saving the object first time

There is no need to override get_readonly_fields. Simplest solution would be:

class ItemAdmin(admin.ModelAdmin):
    exclude=("headline ",)
    readonly_fields=('headline', )

When using readonly_fields you can't override get_readonly_fields, because default implementation reads readonly_fields variable. So overriding it only if you have to have some logic on deciding which field should be read-only at time.

If you want to make the field read-only during creation you should do it the other way round:

def get_readonly_fields(self, request, obj=None):
    if obj is None:
        return ['headline']
    return []