Sign up loner, somehow (5)
I believe the answer is:
enrol
'sign up' is the definition.
(I've seen this before)
'loner somehow' is the wordplay.
'somehow' indicates anagramming the letters.
'loner' with letters rearranged 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" .)