-- with myVar as (select "any value really")
--         then, to get access to the value stored in this construction, you do
--             (select * from myVar)
-- for example

with var as (select 123)    
... where id = (select * from var)

-- You could event re-cast the value (e.g. from INT to VARCHAR) 
with var as (select 123)    
... where id = ((select * from var)::VARCHAR)