Loner ordered to join army? (5)
I believe the answer is:
enrol
'join army?' is the definition.
(as in enrolling in the army)
'loner ordered' is the wordplay.
'ordered' indicates an anagram.
'loner' anagrammed gives 'ENROL'.
'to' acts as a link.
(Other definitions for enrol that I've seen before include "Become a member" , "Enter" , "Register officially" , "Sign up (member)" , "Record" .)