Joined up (8)
I believe the answer is:
enlisted
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">joined up</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 enlisted that I've seen before include "Listened (anag.)" , "Signed up" , "Lower ranks in US army" , "Belonging to (US) armed forces" , "Persuaded to serve" .)