I've used a smear of silicone grease before.

   - Andy.

unless it is perfectly sealed, it is often easier to leave it loose and to let the water out with a hole at a low point, perhaps with a swizzle of rock-wool or similar to discourage small insects.

The other approach is install the clipping near right angles in a shape that allows the tube to distort to a slight  Z or S as it expands or contracts, or to have a section in flexi as bellows.

However, these sort of things

https://cpc.farnell.com/pro-power/ec20-black/expansion-coupler-20mm-x-125mm-black/dp/cbbr5153

work perfectly well if the un-glued end is facing down, or horizontal but under something acting as a 'hat' to keep rain off.

you could fill it with vaseline.....

Mike.

PS

For steel the coefficient is 12E-6 per degree so for your 10m length changing in temperature from say 0 deg C to 40 deg C that 10m would expand by just over 4 mm if allowed to. For PVC the coefficient is more like  7E-5 so the same 10m length would expand by 2.4 cm, say  6 times as much.

Cheers guys.

I suppose I could drill a drain hole at the coupler as well.

Thanks for the advice

Gary

I'm much more used to metal - so this is guess work on my part - Plastic flexible conduit fits perfectly into a plastic through box, so perhaps two though boxes with a bit of flexible conduit glued in might work well. Obviously dont fix the through boxes to the wall. All joints glued as normal.