The two approaches that come to mind are to create two linked tables one
with the field you want to allow changes and all the other fields in the
other. Secure the second table using user level security.
Another approach (far less secure) is to hid the table and all forms
except one and on that form only allow edits to that one field using field
properties.
By implementing User-level security, you can deny direct permission on the
tables, allowing access only through forms. In a form you can easily lock
any part of it that you wish. See the security FAQ: