A wall charger or a USB battery battery pack can let you charge the player with the display off and listen to it at the same time if you want. You can charge it via pc with the display off and listen to it at the same time if you want by using a power only usb cable. These usually come with a usb battery pack. The three usb battery packs I have(Duracell Instant Power, Tekkeon mp1550, and Lenmar powerport mini) all came with a power only USB to mini usb cable that works fine with the Clip+.
If you are interested in usb battery packs, you should post in some of these threads. The Clip+ battery is tiny at only 290 mah. The small Duracell Instant power(it is larger than the Clip+ though) should probably be good for at least 2 full charges of the Clip+ battery(it is rated at 1150 ma, however the charging process is not that efficient). The Lenmar Powerport mini is probably good for just over one full charge(its battery is 650 ma) The Powerport mini is basically the same size as the Clip+. It is a good idea to have at least one usb battery pack, if for no other reason to get a power only cable(the one that comes with the Duracell Instant Power is very short though).
I hope you are using mp3 files on your Clip+, as other formats will give you much less battery life. I guess I get around 12 hours of battery life on my Clip+ using mp3 files. Some have reported battery life of perhaps 7 hours or less using FLAC.