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 anagramming the letters.
'loner' anagrammed gives 'ENROL'.
'to' acts as a 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" .)