Force loner to sign up (5)
I believe the answer is:
enrol
'sign up' is the definition.
(I've seen this before)
'force loner' is the wordplay.
'force' indicates an anagram (letters must be forced into a new order).
'loner' is an anagram of 'ENROL'.
'to' is the link.
(Other definitions for enrol that I've seen before include "Add to list of members" , "Enter" , "Record" , "Insert in register" , "Sign up (member)" .)