This may be a bit of a long shot, but I suspect you want an HTML select element containing a list of all the available clients along with their ids in the 'subfund' form:
<select name="client_id" size="1">
<?
$clients = mysql_query("select client_id, name from clients order by name");
while ($row = mysql_fetch_assoc($result))
{
echo("<option value=\"" . $row['client_id'] . "\">" . $row['name'] . "</option>\n");
}
?>
</select>
If you insert this code into your 'subfund' form, you'll have a list of clients for the user to select and, when the form is submitted, the value $_GET['client_id'] will be the id of the client he selected.
You can then construct your query like this:
$query="insert into subfund values(NULL, $subfund, $manager, " . $_GET['client_id'] . ")";
But I may have got the wrong end of the stick!