Join the armed forces (6)
I believe the answer is:
enlist
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">join the armed forces</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 enlist that I've seen before include "Join up - tinsel (anag)" , "Listen (anag.)" , "Engage" , "Draft" , "Join up, sign on" .)