Loner ordered to join up (5)
I believe the answer is:
enrol
'join up' is the definition.
(I've seen this before)
'loner ordered' is the wordplay.
'ordered' indicates an anagram.
'loner' is an anagram of 'ENROL'.
'to' is the link.
(Other definitions for enrol that I've seen before include "Recruit" , "Add to list of members" , "Register officially" , "Become a member" , "Join up, sign on" .)