Prova cosi:
codice:
<head>
<script>
function Taglia(){
var campo = document.getElementById('suggest1')
var tronca = campo.value.split(',')[0]
campo.value=tronca;
document.forms[0].submit();
return true;
}
</head>
<body>
<form name="myForm" action="tua_pagina_destinazione.xxx" onsubmit="Taglia();return false;" >
<INPUT TYPE="text" id="suggest1" class="suggest_table {th : ['Cognome', 'Nome', 'Residenza', 'CF']}\" alt="table.php"  name="Suggest" />
</form>
</body>