// are you sure these values are correct and the code block is executed at all?
// note a slight change in your conditional, just my preference
if ( ($mode=="suremoveclan") && is_admin($admin) ){
// are you certain these POST vars exist and are valid?
// use var_dump($_POST) in your script someplace
$clan=$_POST['clan'];
$division=$_POST['division'];
$query="DELETE FROM leaguestats_clan_$division WHERE clantag='$clan'";
mysql_query($query)
OR die("Query 1 error:<br />{$query}<br />" .mysql_error());
// does your user have permission to drop the table?
$query2="DROP TABLE leaguestats_$clan";
mysql_query($query2)
OR die("Query 2 error:<br />{$query2}<br />" .mysql_error());
}