I'm wanting the clutch mechanism available on the 10 speed XT derailleur, as chain slap is pretty bad and currently have 9 speed XT setup (cassette,chain,derailleur,changer). Is it possible to use the 10 sp derailleur on the 9 speed?
so long as you fit the 10sp shifter and cassette the chain will be fine...
There is a bodge with SRAM shimano mix, there are some very good deals on 10 sp these days
Won't work.
The derailleurs between 9 and 10 speed have different pull ratios, so with the 9 speed shifter the derailleur won't ever shift correctly.
Similarly, if you try to use only 9 speeds with a 10 speed shifter you'll get shifting issues because the 10 speed increments are smaller than the 9 (keep in mind that the 9 and 10 speed cassettes are the same width, so it follows that on the 10 the sprockets are closer together).
IIRC there's a bodge for Shimano 9.5 speed involving drilling a hole in the mech arm, but I never looked into it much. People said things like "works pretty well" but that doesn't really cut it.
SRAM 9 speed shifter and shimano 10 speed mech is easy and works flawlessly but only really makes sense if you have the shifter.
If I was you, I'd put more inner tube on the chainstay, and replace the whole lot together when it's worn.