AFAIK, you can't use them directly.
Your options are repeating the subselect:
Select
a.field1,
a.field2,
b.field1,
b.field2,
c.field3,
c.field3,
( select aa.field1 from zz inner join aa on zz.field2 = aa.field3 where zz.field3 = a.field1) as field7
FROM
All the tables and joins
WHERE
( select aa.field1 from zz inner join aa on zz.field2 = aa.field3 where zz.field3 = a.field1) = somevalue;
Or making everything a sub select:
select result.* from
(Select
a.field1,
a.field2,
b.field1,
b.field2,
c.field3,
c.field3,
( select aa.field1 from zz inner join aa on zz.field2 = aa.field3 where zz.field3 = a.field1) as field7
FROM
All the tables and joins
WHERE {some stuff}) as result
where result.field7 = somevalue