Deceive or betray (6-5)
I believe the answer is:
double-cross
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">deceive</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >'<span class="explanation-format__original">betray</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(doubling cross is a kind of betraying)</span><br/ ><br/ ></span>
(Other definitions for double-cross that I've seen before include "Deceive supposed ally" , "Cheat - betray" , "Deceive (an ally, maybe)" , "Sell down the river" , "Deceive person one is supposedly helping" .)