Okay, how to clear a sequence:
1. Press REC then hit REST a couple of times (or as may times as the step length you wish the pattern). You now have an empty pattern the length of the number of rests you put in.
2. If your pattern is already the length you wish it to be, press PLAY then press REC so that your pattern is looping around. Press and hold REST for the duration of the loop. You now can punch-in your notes/chords to the pattern (just you may want to run a metronome click from another device so as to indicate the beginning of the loop).
A clear pattern function would be nice though, with some way to specify a default pattern length in MCC perhaps.