jQuery clear input text on focus

To answer your focus question, yes you can do that:

$("input").focus(function() {
  this.value = "";
});

To answer the only allow letters question, this has been asked before.

To filter the input, use

​$('input').on('keydown', function(e) {
    if( !/[a-z]|[A-Z]/.test( String.fromCharCode( e.which ) ) )
        return false;
});​​​​​​​​

To clear the input field on click & focus, use

$('input').on('click focusin', function() {
    this.value = '';
});

Be aware of that this event will fire twice, when you click into a non-focused input control in its current form.

Demo: http://jsfiddle.net/xbeR2/