Comrade in arms (4)
I believe the answer is:
ally
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">comrade in arms</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for ally that I've seen before include "Friend in time of need" , "Cooperating country" , "Colleague, confederate" , "Band together" , "often helpful when cluing adverbs" .)