First of all close the Explorer tab from the View menu, if it is enabled. Next, click on the Document selector tab and then double click the Clip library tab to undock it. Drag it to the centre and let go. Drag it back towards the left hand side, and when you see the group of 5 arrows revealed, position the cursor on the down arrow and release. Viola!
Thank you for your very timely support. I have been trying to get my TextPad 5.03 to go back to the docked document selector and clip library for three weeks. Getting over eager with the mouse undocked both tools and I was wasting time trying to work the old way.