Command-Z, on a Mac (4)
I believe the answer is:
undo
'command-z on a mac' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for undo that I've seen before include "Cancel -- annul -- reverse" , "Dismantle - destroy" , "Overturn" , "set free" , "Cause to become loose" .)