Ctrl-Z command (4)
I believe the answer is:
undo
'ctrl-z command' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for undo that I've seen before include "Unfasten or annul" , "Open button or ruin" , "Ruin, destroy" , "Cause to become loose" , "Cause downfall" .)