I always declutch, go to 1st, wait a sec for the parts to stop spinning, then go to reverse. If you do that and it's still clashing, I would think that means some torque is still being transfered to the input shaft. I can only think of to ways than can happen when the clutch pedal is depressed: 1) Pilot bushing is slightly grabbing the input shaft. 2) Clutch is not adjusted properly, and is not fully dissengaging.
I'm sure someone who knows more than me will chime in, but that's my $.02.
I'm sure someone who knows more than me will chime in, but that's my $.02.