I'm afraid Jetpack doesn't offer such option. The clear button is less and less common in forms, for a few different reasons. You can read more about it here:
http://www.456bereastreet.com/archive/200909/almost_never_add_a_reset_button_to_a_form/
http://www.nngroup.com/articles/reset-and-cancel-buttons/
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Jeremy Herve on "[Plugin: Jetpack by WordPress.com] Clear Button"
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