Reverse in round one? (4)
I believe the answer is:
undo
'reverse' is the definition.
(I know that undo is a kind of reverse)
'in round one?' is the wordplay.
'in' says the answer is hidden in the clue.
'UNDO' is hidden in the letters of 'round one'.
(Other definitions for undo that I've seen before include "Wreck" , "Free from ties, laces, etc" , "Loosen; cancel" , "Cancel -- annul -- reverse" , "Ruin or loosen ties" .)