It shows you’re wrong on both scores – and cheat (6-5)

I believe the answer is:

double-cross

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'cheat' is the definition.
(thesaurus)

I can't explain the rest of the clue.

(Other definitions for double-cross that I've seen before include "Cheat - betray" , "Betray - XX?" , "Deceive person one is supposedly helping" , "Deceive supposed ally" , "Deceive, betray" .)

I've seen this clue in the Evening Standard.

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