Enrol (6)
I believe the answer is:
enlist
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">enrol</span>' is the definition.<br/ ><span class="explanation-format__explanation">(enlisting is a kind of enrolling)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for enlist that I've seen before include "Enrol in the services" , "Join the army" , "Engage" , "Join up, sign on" , "Join the armed services" .)