function clear_search(searchField) {
    if(searchField.value == 'Trefwoord') {
        searchField.value = '';
    }
}

function init() {
    var clearTarget = document.getElementById("avb-search");
    if(clearTarget != null) {
        clearTarget.onclick = function() { 
            this.value=''; 
        }
    }
}
window.onload=init;