Hi TestUser,
Without understanding your scenario (the problem you are trying to solve), it's hard to give you the most pertinent advice ...
You can save your single field to a Web Service that maps to a database. We provide a free trial Web Service that allows you to specify the mapping. You would only map one field and then on submit only that field would be saved. No coding required and you don't have to change security permissions of your form. But, your users must have access to the server that the Web Service is installed on.
You can also save a single field to a text file but that will require writing code and creating a Full Trust form (since writing to the local file system requires elevating default security privileges).