Loner forced to sign up (5)
I believe the answer is:
enrol
'sign up' is the definition.
(I've seen this before)
'loner forced' is the wordplay.
'forced' indicates anagramming the letters.
'loner' is an anagram of 'ENROL'.
'to' is the link.
(Other definitions for enrol that I've seen before include "Recruit" , "Join as member" , "Add to list of members" , "Enter" , "Join up, sing on" .)