Sorry - I assumed you had built it with CSSMenuWriter - as you posted this in the Menu Writer forum.
Forget about clearing the header, - (but I do recommend making the menu an 'include' if you can). No worries if not.
To change the overflow, click inside the header, and on the elements bar - just above the properties panel (inspector) - select < div#header >.
Then, in the CSS Styles panel, change the property 'overflow' from hidden to visible.