I didn't think there was a fuel return pipe on the carb, just a feed. There is a return on the pump. This is BIS language of course, air cooled might be different.
Still doesn't make any difference, if it is falling out you might be able to secure it using one of the Loctite products - similar to threadlock but there are some stronger grades.
I would then suggest a secondary method of stopping it falling out if the loctite doesn't hold it though. These brass fittings falling out of carbs was a major reason for Reliant scimitarts catching fire, so what people do is use either locking wire or (in my case) cable ties that go round the pipe and jubilee clip and then stretch round some part of the carb, in order to make sure the whole thing is always being pulled into the carb body.
Gav
