The most probable cause is the clutch is dirty, full of dust. Take it apart, clean it out and the plates with brake cleaner and reassemble. It's not difficult. 30 minutes tops.
The only other possiblity is the dished drive plate in the clutch has flattened off. It'll be towards the back of the pack, and is denoted by a dot punch in it's face near the cutout in it's periphery. Lay it on a flat surface and check if it's still dished. If it's anywhere near being flat, replace it with a new one for a few £'s.
Don't know where you are in the country. But if it is the dished plate. Don't go to P&H in Crawley. Was in there 20 mins trying to explain what it is, they only wanted to sell me the full clutch pack (knobs). Ended up leaving as the parts dept are useless. Go to someone like Nelly or any good specialist.
I have a similar problem on mine, only does it in first as i pull off and only if I feather it for too long & don't bring the revs up enough (no such problems in any other gears). Same sound though... let us know if you get it sorted mate, hope its nothing serious.
In the mean time I'd have it apart and check/clean it as Nelly says
I think you all need some loud cans I've got a 900 and a 748 and can only just hear the rattle from the clutch over the noise of the cans, blow me if I could tell if it had a delicate grinding noise