Sorry I probably didn't explain myself very well.
I basically want to copy and paste a drill-down dimension as a new drill-down dimension (while removing just one expression). Maybe it's not possible to do it.
The problem I have is that I have so many expressions in the drill-down (as a result of filtering/relationship issues which won't be solved until next week best case scenario) it'll take a while to copy and paste each expression across.
I guess what I can do is open up the app in 2 windows and copy and paste it that way.
I opened the app in a seperate window and copy&pasted everything over without the expression I wanted to leave out and now it works perfectly. I'll close this now.