I should have told you about package versions. The latest version of clamtk is 4.45-1 (testing). So the later versions must still be classified as "unstable". You can view these versions in synaptic by clicking on clamtk, then if you have the properties window showing down the bottom, select the "version" tab (if not select properties from the context menu by right clicking on clamTK). You will see all the versions shown here, if you wish you can select a later "testing" version and then go to the "packages" menu item and select "force version...". BUT it's not advisable because not only are the packages still being tested but sometime the dependencies are all out of sync and then you end up with a broken package!