Defeat in Round One (4)

I believe the answer is:

undo

'defeat' is the definition.
(thesaurus)

'in round one' is the wordplay.
'in' indicates a hidden word.
'UNDO' can be found hidden inside 'round one'.

(Other definitions for undo that I've seen before include "Cancel or reverse an action" , "Open button or ruin" , "Unfasten, loosen" , "Ruin or undermine" , "Release button, knot etc" .)

I've seen this clue in The Sun.

Want a hint initially instead of a full solution? Install my app