I have an extended but otherwise standard arm on the clutch, and it is not really coping!
Where the clutch cable nipple slots into the fitting, the fitting sometimes gives and the cable escapes.
I had a cable tie around it which actually solved the problem, but was just wondering if there were any other ideas?
It's a bit fiddly under there!

Is the fitting still there, as on my old engine i sold it snapped off, but never had any problems keeping the cable attached

maybe you have to much free play on the cable?

you shouldn't be putting the clutch arm under any more force than if it were being operated on the bike.

Is it?

If it is then I'd get the clutch stop adjusted on your pedal.

That is a good point, the engine has only just been put in so I will check play and stop as obviously different clutch!
However, it just looks like actual clip is tired. I was wondering about some clamp or something, otherwise I'll just use the cable tie again.

I would hazard a guess at overtravel from the pedal, cant see any other reason for it, I have had several R1's in bike form and never heard of anything like this.