Goner undone by one cut (6)
I believe the answer is:
ignore
'cut' is the definition.
('ignore' can be a synonym of 'cut')
'goner undone by one' is the wordplay.
'undone' indicates an anagram.
'by' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'goner' with letters rearranged gives 'gnore'.
'gnore' after 'i' is 'IGNORE'.
(Other definitions for ignore that I've seen before include "Pay no attention to, disregard" , "Brush off" , "Deliberately take no notice" , "Pass over" , "Intentionally disregard" .)