Military alliance (4)
I believe the answer is:
nato
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">military alliance</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 nato that I've seen before include "Military alliance, formed 1949" , "International organisation for collective security" , "Transatlantic alliance" , "Military/political alliance" , "Atlantic alliance (abbr.)" .)