Canon EOS-1DX Instruction Manual page 271

Since the editing is performed in 1-sec. increments (position indicated by
[X]), the exact position where the movie is edited may differ slightly from
the position you specified.
If the card does not have enough free space, [New file] will not be
available.
X Editing the Movie's First and Last Scenes
4
Save the movie.
Select [W], then press <0>.
The save screen will appear.
To save it as a new movie, select
[New file]. To save it and overwrite
the original movie file, select
[Overwrite]. Then press <0>.
On the confirmation screen, select
[OK], then press <0> to save the
edited movie and return to the movie
playback screen.
271