Cannot delete clef change at start of system
True for both 3.0.5 and current master:
1) default score, or new score from treble template with line break somewhere
2) add bass clef to first bar of second system
3) select the new clef
Result: nothing happens. Technically, this is the generated clef, the "real" clef is the apparent courtesy on the previous system (that's because clef changes do in fact normally appear before the barline).
Workaround is to delete the "real" clef at the end of the previous system (assuming you didn't disable courtesy clefs).