x-disown-selection-internal

x-disown-selection-internal is a built-in function in `C source code'.

(x-disown-selection-internal SELECTION &optional TIME-OBJECT TERMINAL)

If we own the selection SELECTION, disown it.
Disowning it means there is no such selection.

Sets the last-change time for the selection to TIME-OBJECT (by default
the time of the last event).

TERMINAL should be a terminal object or a frame specifying the X
server to query. If omitted or nil, that stands for the selected
frame's display, or the first available X display.

On Nextstep, the TIME-OBJECT and TERMINAL arguments are unused.
On MS-DOS, all this does is return non-nil if we own the selection.