Reverse the effects of (4)
I believe the answer is:
undo
'reverse the effects of' 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 "Unravel or ruin" , "set free" , "Unfasten - destroy" , "Take back" , "Unfasten or untie" .)