my auto siphon did not come with a clamp either and it is rather frustrating to have the tube slide instead of pumping the wort, or whatever you are transferring. a small spring loaded clamp might work but remember you are working with glass.
the 3-4 days does seem short.
I was told that secondary fermentation not only clears the beer and by going to a smaller vessel like 6 gallons to 5, is a very good idea , because there is less air space involved.