Loner forced to join up (5)
I believe the answer is:
enrol
'join up' is the definition.
(I've seen this before)
'loner forced' is the wordplay.
'forced' is an anagram indicator.
'loner' with letters rearranged gives 'ENROL'.
'to' acts as a link.
(Other definitions for enrol that I've seen before include "Register (as member)" , "Recruit" , "Bring into service" , "Register officially" , "Loner (anag.)" .)