Took apart (5)
'took apart' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for undid that I've seen before include "Loosened; reversed" , "Annulled; unfastened" , "Reversed, destroyed" , "Cancelled - destroyed" , "solved knotty problem" .)
'took apart' is the definition.
('dismantle' can be a synonym of 'take apart')
This is the entire clue.
(Other definitions for dismantled that I've seen before include "Taken to pieces" , "Taken to bits" .)