Alliance acronym (4)
I believe the answer is:
nato
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">alliance acronym</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 nato that I've seen before include "Collective security alliance engaged in Bosnia" , "Military bloc" , "Security agreement" , "Western mil. alliance" , "Military alliance, formed 1949" .)