How to get single value from this MySQL query in PHP

This is how you should do it using mysqli

$query = 'SELECT cname FROM Clients WHERE clientid = ?';

if($stmt = $mysqli->prepare($query)){
    $stmt->bind_param('i', $_POST['clientid']);
    $stmt->execute();
    $stmt->bind_result($cname);
    $stmt->fetch();

    echo $cname; // (optional, prints it out)
}

$clientId=$row["clientid"];
        $sql2="select cname from Clients where clientid='$clientId'";
        //$result = $conn->query($sql2);
        $result = mysql_query($sql2);
        foreach($result->result() as $row){
         echo $row->cname;
}

In order to use the result you need to fetch from it. E.g.:

$result = mysql_query($sql2);
$row = mysql_fetch_array($result);
$cname = $row[0];

mysql deprecated

You are applying wrong methods.

Your code should look like :

<?php
$con=mysqli_connect("localhost","my_user","my_password","my_db");
// Check connection
if (mysqli_connect_errno())
{
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
  }

$sql="SELECT Lastname,Age FROM Persons ORDER BY Lastname";
$result=mysqli_query($con,$sql)

// Numeric array
$row=mysqli_fetch_array($result,MYSQLI_NUM);
printf ("%s (%s)\n",$row[0],$row[1]);

// Associative array
$row=mysqli_fetch_array($result,MYSQLI_ASSOC);
printf ("%s (%s)\n",$row["Lastname"],$row["Age"]);

// Free result set
mysqli_free_result($result);

mysqli_close($con);
    ?> 

Also ready more >> http://www.w3schools.com/php/func_mysqli_fetch_array.asp

Try this way and fix the typo on your sql query use (`) mark for column names, but first see documentation for this deprecated API, instead use PDO or MYSQLi.see mysql_query

$clientId=$row["clientid"];
$sql2="select `cname` from Clients where clientid='$clientId'";
$result = mysql_query($sql2);
$row = mysql_fetch_assoc($result);
//use column name,as you fetch result as associative array
$canme = $row['cname']; 
echo $canme;

N.B: Use PDO or MYSQLi, for prepared statement that will fix your SQL INJECTION related Problems