I bought a second-hand 98 DR200SE. If I let the clutch out suddenly while revving the bike, instead of spinning the rear wheel, the clutch makes a loud squealing sort of sound and slips. It seems to function perfectly under all other conditions. There is no crawl with the clutch disengaged and there seems to be the appropriate amount of play in the cable. Is the clutch dying or does it just need to be adjusted?

It may be that the clutch springs are on the way out...
That or the disc's may be glazed abit.
Hard to say w/o inspecting them...if everything else is working OK...run it till it slips all the time.

You need a clutch. No biggie. It is easy to do.