Set current clipboard greyed out

I'd guess that this is because you are working in Multitrack, where selecting clipboards isn't an option; this only works in Waveform view. What the clipboards do is enable you to save a selection of 'real' audio you've made, so that it can be pasted back to a different place in another 'real' file.

In Multitrack view, it's different. The clips you have aren't real audio at all; they are just markers for where audio would be played from if you put the cursor on them and hit Play. What they do is to indicate which part of the file is to be played. To manipulate these, you don't need clipboards at all - you just move them. If you want to keep them from playing, then move them to a track that you've muted - then they are still there, and can be returned if you need them. With the Waveform view Clipboard way of working, it's rather limited because there are only five of them - but you can put as many clips as you like aside temporarily in Multitrack view.

Essentially, Waveform view is linear editing - on real files, with permanent results if you save them. Multitrack is non-linear; you can put things where you like, and change your mind about what you are doing, almost limitlessly.