Sign up confused loner (5)
I believe the answer is:
enrol
'sign up' is the definition.
(I've seen this before)
'confused loner' is the wordplay.
'confused' indicates anagramming the letters.
'loner' is an anagram of 'ENROL'.
(Other definitions for enrol that I've seen before include "Recruit" , "register for classes" , "Bring into service" , "Sign on, register" , "Join up, sign on" .)