Вие сте използвали tbl_customers в подзаявката, където не сте имали достъп до нея. Просто трябваше да се присъедините, вместо да използвате where:
SELECT tbl_customers.*,(SELECT SUM(amount) As Amount
FROM
(
SELECT tcc.entry_fees*COUNT(tccc.match_contest_id) as amount
FROM `tbl_cricket_customer_contests` tccc
JOIN `tbl_customers` ON (tccc.customer_id = tbl_customers.id)
LEFT JOIN tbl_cricket_contest_matches tccm on(tccm.id=tccc.match_contest_id)
LEFT JOIN tbl_cricket_contests tcc ON (tcc.id=tccm.contest_id)
GROUP BY tccc.match_contest_id
) As DT) as spendamount
FROM (`tbl_customers`)
WHERE `tbl_customers`.`is_deleted` = 'N'
GROUP BY `tbl_customers`.`id`
ORDER BY `spendamount` DESC
