Register, sign on (5)
I believe the answer is:
enrol
This clue is a double definition.
'register' is the first definition.
(I know this)
'sign on' is the second definition.
(I've seen this before)
(Other definitions for enrol that I've seen before include "Loner inclined to join in" , "register for classes" , "Recruit" , "Register officially" , "Register (as student)" .)