Want to use In derived column

i have one table i.e Fact

col 1 

col 2 ---> ISNULL((SELECT DISTINCT PT2 FROM dbo.Class as CL WHERE P.PNo = CL.PNo AND P.CNo = CL.CNo AND CL.C = P.C AND CL.D = PTP.D), 'ABC') AS P_T_D

col 3

i am so confuse that how to set this select condition for on column in derived column 

let me know if anyone have any idea how to set 

thanks