SN2 on compound containing halogen and carbonyl
If it attacks the carbonyl carbon, then you form an unstable tetrahedral intermediate with a negative formal charge on the oxygen. Unless you treat the compound with water so that the oxygen can remove it's formal charge, what will happen is that the reaction goes backwards where the CN group is kicked off.
However, if the CN attacks the Cl group, then you can have a substitution where you get a stable compound finally.