Knockout (4)

I think it could be either:

lulu

kayo

'knockout' is the definition.
(I've seen this before)

This is all the clue.

(Other definitions for kayo that I've seen before include "Fight's ending" , "floor" , "Stun" , "Decisive end to a fight?" , "KO (sl.)" .)

I've seen this clue in The Guardian, The Telegraph and The Times.

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