Loosen, as a necktie (4)
I believe the answer is:
undo
'loosen as a necktie' 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 "Ruin or loose" , "Loosen; annul" , "Loosen or reverse" , "Destroy - nullify" , "Disentangle" .)