Joined by treaty (6)
I believe the answer is:
allied
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">joined by treaty</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for allied that I've seen before include "Owl's cry" , "in concert" , "(Of nations) working together" , "Joined by, eg, treaty" , "United through treaty" .)