Sign up loner to go travelling (5)
I believe the answer is:
enrol
'sign up' is the definition.
(I've seen this before)
'loner to go travelling' is the wordplay.
'to go travelling' is an anagram indicator (letters travel around).
'loner' is an anagram of 'ENROL'.
(Other definitions for enrol that I've seen before include "register for classes" , "Join up, sign on" , "Bring into service" , "Loner (anag.)" , "Enter" .)