Usually select just goes to PRIMARY selection. To make it also go to CLIPBOARD selection add this to ~/.Xresources file:
XTerm*VT100.translations: #override : select-end(PRIMARY, CLIPBOARD, CUT_BUFFER0)
To test this without restarting X do
xrdb -merge /home/acorn/.Xresources
You can also start an xterm with this behavior without touching .Xresources
with the command
xterm -xrm 'XTerm*VT100.translations: #override : select-end(PRIMARY, CLIPBOARD, CUT_BUFFER0)'
This is from
http://www.davidsimmons.com/soft/xtermhacks/.