Something like...
SELECT c.c_id, c.c_name, SUM( p.p_sum ) AS sum
FROM clients c, contracts_rent r, payments p
WHERE c.c_id = r.c_id
AND r.contract_id = p.contract_id
GROUP BY c.c_id
HAVING SUM( p.p_sum ) > 1300
When you want to filter on the aggregate function value, you use the HAVING keyword