Register, sign on (5)
I believe the answer is:
enrol
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">register</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know this)</span><br/ ><br/ >'<span class="explanation-format__original">sign on</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for enrol that I've seen before include "Loner inclined to join in" , "register for classes" , "Recruit" , "Register officially" , "Register (as student)" .)