SELECT SUM() FROM (SELECT (SELECT ())

There is absolutely no problem whatsoever to achieve what you want. We don't see your entier query, but the most common problem is people forget to add an alias to their nested select statement. Take a look at this sample that works perfectly :

select sum(col1) as sum1 
from ( select col1 
        from ( select 1 col1 union all select 2 union all select 3 ) tmp 
     ) tmp2

According to the OP, here is your final query :

SELECT  SUM(numbers)
FROM    (
            SELECT  columnA
                    AS
                   'numbers'
              FROM    tableA
             WHERE   clause
        ) tmp

You need an alias on the subquery:

SELECT  SUM(numbers)
FROM    
(
    script  -- your subquery will go here
) src   -- place an alias here

So your full query will be:

select sum(numbers)
from
(
   SELECT  columnA  AS numbers
   FROM    tableA
   WHERE   clause
) src