Cheat, betray (11)
I believe this is a double definition.
'cheat' is the first definition.
(thesaurus)
'betray' is the second definition.
(doubling cross is a kind of betraying)
(Other definitions for double-cross that I've seen before include "Cheat - betray" , "Deceive (an ally, maybe)" , "Deceive supposed ally" , "Sell down the river" , "Betray - XX?" .)