how to insert font awesome icon

I’m Japanese and English is unskillful.
Even if there is impolite way to fill out, please don’t worry about it.

The one I want to tell would like to add “fa fa-bars” to the highest rank item at the following tag part, is it possible to add icon to only the highest rank item?

// Font Awesome icons
if ( ! empty( $icon ) )
	$item_output .= '<a'. $attributes .'><span class="fa ' . esc_attr( $icon ) . '"></span>&nbsp;';
else
	$item_output .= '<a'. $attributes .'>';