International security organisation (4)
I believe the answer is:
nato
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">international security organisation</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/political alliance" , "Military alliance (acronym)" , "Western mil. alliance" , "N. Atlantic alliance" , "US/Europe alliance" .)