i bought a used 05 550 polaris super sport with 1,200 miles on it.  an i noticed just recently when you drive the machine at 5-10 mph you hear a clunk like something is slipping and sometimes when put into reverse you give it gas and it feels like someone is holding the break.  what could cause this  chain in the clutch?  any thoughts?

If the sled sat i would guess there is some corrosion on the clutch shaft causing the clunk....and the you probably smoked the belt running it on a high friction surface like your lawn.

I'd look at the chaincase... if the tensioner is broken that could cause some of those symptoms.

Also look over the clutches, is the spider/rollers/weights in the primary in good condition, and is the helix/rollers/spring in the secondary clean?  Probably time for a new belt too.