Cancel during sundown (4)
I believe the answer is:
undo
'cancel' is the definition.
(I've seen this before)
'during sundown' is the wordplay.
'during' says the answer is hidden in the clue.
'UNDO' can be found hidden inside 'sundown'.
(Other definitions for undo that I've seen before include "Ruin prospects of; cancel" , "Unfasten or annul" , "Ruin or loosen ties" , "Destroy - nullify" , "Dissolve - ruin" .)