Changing the belt will have no effect on the clutch adjustment.
If the clutch is slipping, on older types (triangular in shape), some adjustment cane be made to the 3 bolts holding the assembly together to increase the clamping force, but generally, when they have worn to the extent of requiring adjustment, the're on borrowed time anyway. About £120 to replace.
phil_saunders(a)bigfoot.com
Phil Saunders, October 2012
