Based on what condition you want to search the record from "x.x" object? There should be some condition to fetch the single record out of hundreds or thousands of records.

class MyClass2(models.Model):
    _inherit = 'z.z'

    @api.model
    def create(self, vals):
        field1 = self.env['x.x'].search([('field1', '=', 'Some Value')], limit=1).field1
        vals.update({'field2': field1})
        return super(MyClass2, self).create(vals)

In the example, [('field1', '=', 'Some Value')] is a condition (domain) to search the record from another object.