Usa left join en lugar de inner join quedando algo como esto:
Código SQL:
Ver originalSELECT t3.* FROM clientes_pago AS t1
LEFT JOIN pagos AS t2 ON t1.idpago=t2.id_pago
LEFT JOIN clientes AS t3 ON t1.id_cliente=t3.idcliente
LEFT JOIN meses AS t4 ON t1.id_mes=t4.id_mes
WHERE mes='enero' AND t1.id_pago IS NULL