Arranged in threes (7)
I believe the answer is:
ternate
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">arranged in threes</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for ternate that I've seen before include "Like some leaves" , "in three parts?" .)