In my experience screw clamps are great for larger hoses but problematic on very small diameters. Even with flanged fuel hose clamps I wouldn't expect to go below about 10mm.
If you want to seal a flexible hose onto a plastic spigot, then the three options I'd consider are:
- Glue. If you can find anything that will bond with both materials, use this on it own on in combination with the clamping below
- Small zip tie. The smallest cable ties I know of have a body about 6mm wide and would work as long as you have a spigot at least that long. I routinely use these for vacuum lines, small bore hoses used for windscreen washers and that sort of thing.
- On shorter spigots you can just use several turns of wire twisted to tighten and lock it. I'd usually only consider this as a temporary fix, but for a low pressure system with only water in it, I don't see any reason not to use it as a permanent solution.
To have any hope of a reliable seal you'd need to support the hose so that the join was never under tension and was, if anything, under slight compression. Also note that the hose must be smaller diameter than the spigot so that it is slightly under tension. Trying to compress a hose down onto a spigot is asking for trouble.