Clementine is a port of Amarok 1.4 to QT 4, giving users an application that fits in well with KDE 4 while still resembling the previous version of the library-based music player. As I've commented in the past, acceptance of KDE 4 wasn't helped by the fact that the developers took it upon themselves to redesign some of the most popular apps such as Konqueror and Amarok.
As Clementine is a fork of the 1.4 branch of Amarok that has been ported to QT 4, users can expect a layout and functionality that replicates the KDE 3.x era of the program alongside updates that ensure that it fits in well with the look of the KDE 4.x desktop environment. To jog the memory of former users, this amounts to a two pane design.
rest here