We don't recommend inserting a knife or any other object into your toaster, whether to clean it, remove a blockage or otherwise. Not only is there a good chance you will damage your toaster, but if switched on you also risk receiving a nasty electric shock!
There are many products advertised as being suitable for heating in toasters, but some we advise against using. Jam filled tarts, or anything with sugar coating should not be toasted. These ingredients can reach extremely high temperatures and risk starting a fire.