Sign up loner, somehow (5)
'sign up' is the definition.
(I've seen this before)
'loner somehow' is the wordplay.
'somehow' indicates an anagram.
'loner' anagrammed gives 'ENROL'.
(Other definitions for enrol that I've seen before include "Sign up (member)", "Join up, sign on", "Add to list of members", "Sign on, register", "Sign on, being a loner".)