Go back on (4)
I believe the answer is:
undo
'go back on' 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 "Cause to become loose" , "Open, a knot say" , "Unfasten or untie" , "Destroy - nullify" , "Cancel, annul; open" .)