Join the military (6)
I believe the answer is:
enlist
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">join the military</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for enlist that I've seen before include "Join up, sign on" , "Join up in silent way" , "Enrol - recruit" , "Join the armed forces" , "augment army" .)