Return things to the way they were during sundown (4)
I believe the answer is:
undo
'return things to the way they were' is the definition.
(reverse some action or effect)
'during sundown' is the wordplay.
'during' indicates the answer is hidden within the clue.
'UNDO' is hidden within 'sundown'.
(Other definitions for undo that I've seen before include "Destroy - nullify" , "Ruin prospects of; cancel" , "Release fastening" , "Unfasten or untie" , "Overturn" .)