.heading a:hover .submenu
                 ^

<li class="heading"><a href="#">Home</a>
     <ul class="submenu">

.heading a:hover .submenu{
    display: block;
}

.heading:hover .submenu{
    display: block;
}

.heading a:hover ~ .submenu{
    display: block;
}

.heading .submenu:hover {
    display: block;
}