Is this the clutch pack on the PTO shaft?
Or the main drive clutch for the tractor's transmission (which would require splitting the entire tractor in half to service)? No small feat.
If it is the drive shaft for the Brush hog, then just track down what has gone bad.
Tractor driving forward and reverse?
PTO from out from tractor working or not?
Shaft through clutch?
Sheer pins?
I had a brush hog that the 90 degree ring and pinion got separated slightly and started slipping.
Good Luck