Loner managed to sign up (5)
I believe the answer is:
enrol
'sign up' is the definition.
(I've seen this in another clue)
'loner managed' is the wordplay.
'managed' indicates an anagram.
'loner' anagrammed gives 'ENROL'.
'to' acts as a link.
(Other definitions for enrol that I've seen before include "Bring into service" , "Recruit" , "Loner (anag.)" , "Sign up (member)" , "Enlist" .)