They're up in arms (5)
I believe the answer is:
ulnae
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">they're up in arms</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for ulnae that I've seen before include "Army parts" , "Thin bones in upper arms" , "parts of limb" .)