Sign up loner, somehow (5)
I believe the answer is:
enrol
'sign up' is the definition.
(I've seen this in another clue)
'loner somehow' is the wordplay.
'somehow' is an anagram indicator.
'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" .)