Loner forced to register (5)
I believe the answer is:
enrol
'register' is the definition.
(I know that enrol is a more specific form of the action register)
'loner forced' is the wordplay.
'forced' indicates anagramming the letters.
'loner' with letters rearranged gives 'ENROL'.
'to' acts as a link.
(Other definitions for enrol that I've seen before include "Bring into service" , "Insert in register" , "Sign on, register" , "Join as member" , "Enter" .)