Loosen, as shoelaces (4)
(Other definitions for undo that I've seen before include "Untie, loosen" , "Dissolve - ruin" , "Unravel or ruin" , "Cancel - destroy" , "Loosen or reverse" .)
'loosen as shoelaces' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for untie that I've seen before include "Undo, unlace" , "Detach" , "Open a knot" , "resolve" , "Set free" .)