Are you sure the clutch is slipping? Because of the design of the forward and reverse clutches, if a Greenfield slips when driving forward but will still drive properly in reverse, the problem is nearly always that the automatic belt tensioner has not done its job properly, and the drive belt is slipping. When this happens the standard cure is to lever the sliding pulley under the seat forward until you hear the tensioner ratchet click into the next position on the slide-rod.
If that is not the problem, it may be that the forward-reverse pedal linkage needs attention or there is wear in the forward clutch (which is used far more than the reverse clutch), but this is less common.