I had what sounds like the same issue a few weeks ago, horrible sounding mechanical crunching when letting the clutch out, seemed to ease up after a little riding. Would go away if I held the clutch in, I was convinced this was a real horrible mechanical issue but turns out my chain was just knackered (almost same as yours about 16500 miles on it), couldn't hold its tension,I would get it tightened up and then be loose again 100 miles later making the same noise.
Looks like your in for a new chain ( might aswell do sprockets etc at same time).
Pictures of the chain and sprocket set might help to confirm though?