XmTextClearSelection(3) on Linux
The text e4de58efb110, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
A Text function that clears the primary selection "XmTextClearSelection" "Text functions" "XmTextClearSelection"