Like Worf said if it's been standing a long time then clutch is prob rusted to flywheel. If you've got access to a field then rag it around for a few minutes with the clutch depressed alternately pressing and releasing the accelerator. You'll have to pull up the floor and take the gearbox out if you can't free it off - so it's probably work risking your rear halfshafts by giving it a bit of a hammering. On the other hand you might end up having to replace a halfshaft and take the box out though if you're too harsh with it.