While technically there is no "Clean Preset" on the Fender® G-DEC® 3, many of users have found that using the Jazz Master amp model setting with the Compressor off and the Timbre set to none will produce a non-effected, neutral sound. Also make sure that the treble, mid, and bass controls are all neutral.
For the Fender® Mustang Series products and the Passport® mini, the '57 Twin Reverb amp model with no Gain or effects is a good place to start for a "Clean" tone. Then you can launch AmpliTube and use your Mustang Series product with this software. For Mustang (V2) Series amps and the Mustang Floor, users can select the Studio Preamp amp model which is intended to be a clean model.