Use derived expression in select statement

I am converting a database from Sybase and have found the following command
does not work in the same fashion.

select dy_c , dy_sect as tsect, (cy_bfwd + dy_p1T4) as curr_bal from fclitot
where dy_yr = 0 and (curr_bal) <-0.005

In the actual command used curr_bal is calculated from a very much longer
expression and rather than repeat the expression I simply refer to it by its
assigned name in sybase.

In postgres it tells me curr_bal is not found. is their a simular shorthand
available in postgres? curr_bal is calc from about 12 fields :{

thanks

Richard