If your talking about the Edit > Crop, command,
(it removes everything outside the current selection,
or if there is no selection the crop toolbar shows up
and one can finetune the position, size and aspect ratio
of the selection before cropping),
the keyboard shortcut for the crop command in
xnview mp v0.61 and xnview 2.04 is
shift + x
If your talking about the Edit > Cut, command
(it copies then deletes the current selection),
the keyboard shortcut for the cut command in
pretty much every application is
ctrl + x
The Edit > Cut or ctrl + x, command does not
work in xnview mp v0.61.
There is a recent thread about it here:
Ctrl+X not available for cutting from picture